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POInteresting Linq to SQL common base class behavior
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copied!<p>It's a question about <strong>InvalidOperationException</strong> with message <em>Class member X is unmapped.</em></p> <p>One of our system has the same <strong>base entity</strong> for each LinqToSql entity with framework version 3.5.</p> <p>I ran into a very strange problem and I started a research about it. I made a very small project to be able to localize the problem more easily.</p> <p><strong>Entity base class</strong></p> <pre><code>public abstract class EntityBase { public virtual long ID { get; set; } } </code></pre> <p><strong>DataContext and Entity</strong></p> <pre><code>[Database(Name = "TestDatabase")] public class EntitiesDataContext : DataContext { public EntitiesDataContext() : base(Settings.Default.TestDatabaseConnectionString, new AttributeMappingSource()) { } } [Table(Name = "dbo.MyEntity")] public class MyEntity : EntityBase { private long _EntityID; [Column(Name = "EntityID", Storage = "_EntityID")] public override long ID { get { return _EntityID; } set { _EntityID = value; } } [Column] public string Title; } </code></pre> <p>The problem is the overriden <strong>ID</strong>. I made a lot of variation with different mapping attribute/property name setups, but it seems the problem is not the naming but the <strong>base class</strong>. And there is also a difference between .NET3.5 and .NET4.0.</p> <p>So, for the following statements, imagine a</p> <pre><code>using (var ctx = new EntitiesDataContext()) { //statement } </code></pre> <p>around.</p> <p>And GetTable() is <code>GetTable<MyEntity>().</code></p> <p><strong>Fails</strong> means <em>Class member EntityBase.ID is unmapped.</em> exception. <strong>Works</strong> means the expected behavior.</p> <p>1 (in 3.5) WORKS:</p> <pre><code>var result = ctx.GetTable().Where(i => i.ID == 2).FirstOrDefault();</code></pre> <p>2 (in 3.5) <strong>FAILS</strong>:</p> <pre><code>var result = ctx.GetTable().FirstOrDefault(i => i.ID == 2);</code></pre> <p>3 (in 3.5) WORKS:</p> <pre><code>var result = ctx.GetTable().FirstOrDefault(i => i.ID.Equals(2));</code></pre> <p>4 (in 3.5) WORKS:</p> <pre><code>var result = ctx.GetTable().Where(i => true).FirstOrDefault(i => i.ID == 2);</code></pre> <p>5 (in 3.5) WORKS:</p> <pre><code>var result = ctx.GetTable().Where(i => i.ID == 2).FirstOrDefault();</code></pre> <p>6 (in 4.0) <strong>FAILS</strong>:</p> <pre><code>var result = ctx.GetTable().Where(i => i.ID == 2).FirstOrDefault()</code></pre> <p>7 (in 4.0) WORKS:</p> <pre><code>var result = ctx.GetTable().Where(i => i.ID.Equals(2)).FirstOrDefault();</code></pre> <p>8 (in 4.0) <strong>FAILS</strong> (redundant with 6)</p> <pre><code>var result = ctx.GetTable().FirstOrDefault(i => i.ID == 2);</code></pre> <p>9 (in 4.0) WORKS:</p> <pre><code>var result = ctx.GetTable().FirstOrDefault(i => i.ID.Equals(2));</code></pre> <p>10 (in 4.0) WORKS:</p> <pre><code>ctx.GetTable().Where(i => true).FirstOrDefault(i => i.ID == 2);</code></pre> <p><strong>So</strong> I cannot figure out why it fails where it fails. Especially, why this works</p> <pre><code>var result = ctx.GetTable().Where(i => true).FirstOrDefault(i => i.ID == 2);</code></pre> <p>if FirstOrDefault with predicate does not? And why Equals works where == is not. </p> <p>I was looking for some Equals and == difference description, but it does not give me the answer for Where(i => true)... thing.</p> <p>It seems that it's <strong>not about the query</strong> but the object initialization. Because:</p> <p>in 4.0 WORKS:</p> <pre><code>var result = ctx.GetTable().Where(i => i.ID == 2).Select(i => i.Title).FirstOrDefault();</code></pre> <p>but :)</p> <p>in 4.0 it's also WORKS:</p> <pre><code>var result = ctx.GetTable().FirstOrDefault();</code></pre> <p>So <strong>maybe not the object initialization</strong>?</p> <p>The SQL being built by LinqToSql is the <strong>same</strong> for == and Equals. Also the <strong>same</strong> for </p> <pre><code>Where(i => true).FirstOrDefault(i => i.ID == 2) </code></pre> <p>and</p> <pre><code>FirstOrDefault(i => i.ID == 2) </code></pre> <p>There is no SQL query until FirstOrDefault, and it builds the query correctly, as it expected. Where(i => true) just continues the expression building and FirstOrDefault predicate included in the SQL queries.</p> <p>I was looking for <strong>other reason in Reflector</strong> in <strong>MSIL</strong>, but found nothing special.</p> <p>Any guess? :)</p> <p>Thank you</p> <p><strong>####Continuation (in .NET4.0)</strong></p> <p>I set up 5 simple methods to easily check in <strong>reflector</strong>:</p> <pre><code>public void WithEquals(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().FirstOrDefault(i => i.ID.Equals(2)); } public void WithFakeWhereAndOperator(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().Where(i => true).FirstOrDefault(i => i.ID == 2); } public void WithOperator(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().FirstOrDefault(i => i.ID == 2); } public void WithOperatorSelect(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().Where(i => i.ID == 2).Select(i => i).FirstOrDefault(); } public void WithOperatorAndWhere(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().Where(i => i.ID == 2).FirstOrDefault(); } </code></pre> <p>WithOperator and WithOperatorAndWhere fails, but here is the <strong>MSIL</strong> and what I see:</p> <p><strong>WithOperator</strong></p> <pre><code> .method public hidebysig instance void WithOperator(class LinqToSqlTest.EntitiesDataContext ctx) cil managed { .maxstack 5 .locals init ( [0] class [System.Core]System.Linq.Expressions.ParameterExpression CS$0$0000, [1] class [System.Core]System.Linq.Expressions.ParameterExpression[] CS$0$0001) L_0000: nop L_0001: ldarg.1 L_0002: callvirt instance class [System.Data.Linq]System.Data.Linq.Table`1<!!0> [System.Data.Linq]System.Data.Linq.DataContext::GetTable<class LinqToSqlTest.MyEntity>() L_0007: ldtoken LinqToSqlTest.MyEntity L_000c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0011: ldstr "i" L_0016: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_001b: stloc.0 L_001c: ldloc.0 L_001d: ldtoken instance int64 LinqToSqlTest.EntityBase::get_ID() L_0022: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_0027: castclass [mscorlib]System.Reflection.MethodInfo L_002c: call class [System.Core]System.Linq.Expressions.MemberExpression [System.Core]System.Linq.Expressions.Expression::Property(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo) L_0031: ldc.i4.2 L_0032: conv.i8 L_0033: box int64 L_0038: ldtoken int64 L_003d: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0042: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_0047: call class [System.Core]System.Linq.Expressions.BinaryExpression [System.Core]System.Linq.Expressions.Expression::Equal(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.Expression) L_004c: ldc.i4.1 L_004d: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_0052: stloc.1 L_0053: ldloc.1 L_0054: ldc.i4.0 L_0055: ldloc.0 L_0056: stelem.ref L_0057: ldloc.1 L_0058: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_005d: call !!0 [System.Core]System.Linq.Queryable::FirstOrDefault<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_0062: pop L_0063: ret } </code></pre> <p><strong>WithFakeWhereAndOperator</strong></p> <pre><code> .method public hidebysig instance void WithFakeWhereAndOperator(class LinqToSqlTest.EntitiesDataContext ctx) cil managed { .maxstack 5 .locals init ( [0] class [System.Core]System.Linq.Expressions.ParameterExpression CS$0$0000, [1] class [System.Core]System.Linq.Expressions.ParameterExpression[] CS$0$0001) L_0000: nop L_0001: ldarg.1 L_0002: callvirt instance class [System.Data.Linq]System.Data.Linq.Table`1<!!0> [System.Data.Linq]System.Data.Linq.DataContext::GetTable<class LinqToSqlTest.MyEntity>() L_0007: ldtoken LinqToSqlTest.MyEntity L_000c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0011: ldstr "i" L_0016: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_001b: stloc.0 L_001c: ldc.i4.1 L_001d: box bool L_0022: ldtoken bool L_0027: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_002c: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_0031: ldc.i4.1 L_0032: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_0037: stloc.1 L_0038: ldloc.1 L_0039: ldc.i4.0 L_003a: ldloc.0 L_003b: stelem.ref L_003c: ldloc.1 L_003d: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_0042: call class [System.Core]System.Linq.IQueryable`1<!!0> [System.Core]System.Linq.Queryable::Where<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_0047: ldtoken LinqToSqlTest.MyEntity L_004c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0051: ldstr "i" L_0056: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_005b: stloc.0 L_005c: ldloc.0 L_005d: ldtoken instance int64 LinqToSqlTest.EntityBase::get_ID() L_0062: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_0067: castclass [mscorlib]System.Reflection.MethodInfo L_006c: call class [System.Core]System.Linq.Expressions.MemberExpression [System.Core]System.Linq.Expressions.Expression::Property(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo) L_0071: ldc.i4.2 L_0072: conv.i8 L_0073: box int64 L_0078: ldtoken int64 L_007d: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0082: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_0087: call class [System.Core]System.Linq.Expressions.BinaryExpression [System.Core]System.Linq.Expressions.Expression::Equal(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.Expression) L_008c: ldc.i4.1 L_008d: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_0092: stloc.1 L_0093: ldloc.1 L_0094: ldc.i4.0 L_0095: ldloc.0 L_0096: stelem.ref L_0097: ldloc.1 L_0098: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_009d: call !!0 [System.Core]System.Linq.Queryable::FirstOrDefault<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_00a2: pop L_00a3: ret } </code></pre> <p>As I see, there is no difference between the <strong>FirstOrDefault</strong> calls, the <strong>WithFakeWhereAndOperator</strong> only 'includes' a few line about the Where statement:</p> <p><img src="https://i.stack.imgur.com/7lwpO.jpg" alt="WithFakeWhereAndOperator and WithOperator difference"></p> <p>And the <strong>WithEquals</strong></p> <pre><code>.method public hidebysig instance void WithEquals(class LinqToSqlTest.EntitiesDataContext ctx) cil managed { .maxstack 7 .locals init ( [0] class [System.Core]System.Linq.Expressions.ParameterExpression CS$0$0000, [1] class [System.Core]System.Linq.Expressions.Expression[] CS$0$0001, [2] class [System.Core]System.Linq.Expressions.ParameterExpression[] CS$0$0002) L_0000: nop L_0001: ldarg.1 L_0002: callvirt instance class [System.Data.Linq]System.Data.Linq.Table`1<!!0> [System.Data.Linq]System.Data.Linq.DataContext::GetTable<class LinqToSqlTest.MyEntity>() L_0007: ldtoken LinqToSqlTest.MyEntity L_000c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0011: ldstr "i" L_0016: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_001b: stloc.0 L_001c: ldloc.0 L_001d: ldtoken instance int64 LinqToSqlTest.EntityBase::get_ID() L_0022: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_0027: castclass [mscorlib]System.Reflection.MethodInfo L_002c: call class [System.Core]System.Linq.Expressions.MemberExpression [System.Core]System.Linq.Expressions.Expression::Property(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo) L_0031: ldtoken instance bool [mscorlib]System.Int64::Equals(int64) L_0036: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_003b: castclass [mscorlib]System.Reflection.MethodInfo L_0040: ldc.i4.1 L_0041: newarr [System.Core]System.Linq.Expressions.Expression L_0046: stloc.1 L_0047: ldloc.1 L_0048: ldc.i4.0 L_0049: ldc.i4.2 L_004a: conv.i8 L_004b: box int64 L_0050: ldtoken int64 L_0055: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_005a: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_005f: stelem.ref L_0060: ldloc.1 L_0061: call class [System.Core]System.Linq.Expressions.MethodCallExpression [System.Core]System.Linq.Expressions.Expression::Call(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo, class [System.Core]System.Linq.Expressions.Expression[]) L_0066: ldc.i4.1 L_0067: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_006c: stloc.2 L_006d: ldloc.2 L_006e: ldc.i4.0 L_006f: ldloc.0 L_0070: stelem.ref L_0071: ldloc.2 L_0072: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_0077: call !!0 [System.Core]System.Linq.Queryable::FirstOrDefault<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_007c: pop L_007d: ret } </code></pre> <p>The difference is bigger:</p> <p><img src="https://i.stack.imgur.com/etLBD.jpg" alt="WithOperator and WithEquals difference"></p> <p>In <strong>WithEquals</strong>, there is an extra </p> <pre><code>[System.Core]System.Linq.Expressions.Expression[] </code></pre> <p>initialization, and it calls Equals in the middle of the method.</p> <p>Also, I can see that <strong>WithOperator</strong> uses </p> <pre><code>call class [System.Core]System.Linq.Expressions.BinaryExpression [System.Core]System.Linq.Expressions.Expression::Equal(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.Expression) </code></pre> <p>while <strong>WithEquals</strong> uses</p> <pre><code>call class [System.Core]System.Linq.Expressions.MethodCallExpression [System.Core]System.Linq.Expressions.Expression::Call(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo, class [System.Core]System.Linq.Expressions.Expression[]) </code></pre> <p>This is in the <strong>line 38</strong> on the second image.</p> <p>Hm, maybe it's a problem about the difference between <strong>BinaryExpression</strong> and <strong>MethodCallExpression</strong>? I will make further research about these.</p> <p><strong>So on</strong></p> <p>We have a working</p> <pre><code>public void WithOperatorSelect(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().Where(i => i.ID == 2).Select(i => i).FirstOrDefault(); } </code></pre> <p>Same as </p> <pre><code>public void WithOperatorAndWhere(EntitiesDataContext ctx) { ctx.GetTable<MyEntity>().Where(i => i.ID == 2).FirstOrDefault(); } </code></pre> <p>but with an extra fake select and it <strong>works</strong>.</p> <p><strong>MSIL</strong></p> <p><strong>WithOperatorAndSelect</strong></p> <pre><code>.method public hidebysig instance void WithOperatorSelect(class LinqToSqlTest.EntitiesDataContext ctx) cil managed { .maxstack 5 .locals init ( [0] class [System.Core]System.Linq.Expressions.ParameterExpression CS$0$0000, [1] class [System.Core]System.Linq.Expressions.ParameterExpression[] CS$0$0001) L_0000: nop L_0001: ldarg.1 L_0002: callvirt instance class [System.Data.Linq]System.Data.Linq.Table`1<!!0> [System.Data.Linq]System.Data.Linq.DataContext::GetTable<class LinqToSqlTest.MyEntity>() L_0007: ldtoken LinqToSqlTest.MyEntity L_000c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0011: ldstr "i" L_0016: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_001b: stloc.0 L_001c: ldloc.0 L_001d: ldtoken instance int64 LinqToSqlTest.EntityBase::get_ID() L_0022: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_0027: castclass [mscorlib]System.Reflection.MethodInfo L_002c: call class [System.Core]System.Linq.Expressions.MemberExpression [System.Core]System.Linq.Expressions.Expression::Property(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo) L_0031: ldc.i4.2 L_0032: conv.i8 L_0033: box int64 L_0038: ldtoken int64 L_003d: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0042: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_0047: call class [System.Core]System.Linq.Expressions.BinaryExpression [System.Core]System.Linq.Expressions.Expression::Equal(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.Expression) L_004c: ldc.i4.1 L_004d: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_0052: stloc.1 L_0053: ldloc.1 L_0054: ldc.i4.0 L_0055: ldloc.0 L_0056: stelem.ref L_0057: ldloc.1 L_0058: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_005d: call class [System.Core]System.Linq.IQueryable`1<!!0> [System.Core]System.Linq.Queryable::Where<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_0062: ldtoken LinqToSqlTest.MyEntity L_0067: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_006c: ldstr "i" L_0071: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_0076: stloc.0 L_0077: ldloc.0 L_0078: ldc.i4.1 L_0079: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_007e: stloc.1 L_007f: ldloc.1 L_0080: ldc.i4.0 L_0081: ldloc.0 L_0082: stelem.ref L_0083: ldloc.1 L_0084: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, class LinqToSqlTest.MyEntity>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_0089: call class [System.Core]System.Linq.IQueryable`1<!!1> [System.Core]System.Linq.Queryable::Select<class LinqToSqlTest.MyEntity, class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, !!1>>) L_008e: call !!0 [System.Core]System.Linq.Queryable::FirstOrDefault<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>) L_0093: pop L_0094: ret } </code></pre> <p><strong>WithOperatorAndWhere</strong></p> <pre><code> .method public hidebysig instance void WithOperatorAndWhere(class LinqToSqlTest.EntitiesDataContext ctx) cil managed { .maxstack 5 .locals init ( [0] class [System.Core]System.Linq.Expressions.ParameterExpression CS$0$0000, [1] class [System.Core]System.Linq.Expressions.ParameterExpression[] CS$0$0001) L_0000: nop L_0001: ldarg.1 L_0002: callvirt instance class [System.Data.Linq]System.Data.Linq.Table`1<!!0> [System.Data.Linq]System.Data.Linq.DataContext::GetTable<class LinqToSqlTest.MyEntity>() L_0007: ldtoken LinqToSqlTest.MyEntity L_000c: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0011: ldstr "i" L_0016: call class [System.Core]System.Linq.Expressions.ParameterExpression [System.Core]System.Linq.Expressions.Expression::Parameter(class [mscorlib]System.Type, string) L_001b: stloc.0 L_001c: ldloc.0 L_001d: ldtoken instance int64 LinqToSqlTest.EntityBase::get_ID() L_0022: call class [mscorlib]System.Reflection.MethodBase [mscorlib]System.Reflection.MethodBase::GetMethodFromHandle(valuetype [mscorlib]System.RuntimeMethodHandle) L_0027: castclass [mscorlib]System.Reflection.MethodInfo L_002c: call class [System.Core]System.Linq.Expressions.MemberExpression [System.Core]System.Linq.Expressions.Expression::Property(class [System.Core]System.Linq.Expressions.Expression, class [mscorlib]System.Reflection.MethodInfo) L_0031: ldc.i4.2 L_0032: conv.i8 L_0033: box int64 L_0038: ldtoken int64 L_003d: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle) L_0042: call class [System.Core]System.Linq.Expressions.ConstantExpression [System.Core]System.Linq.Expressions.Expression::Constant(object, class [mscorlib]System.Type) L_0047: call class [System.Core]System.Linq.Expressions.BinaryExpression [System.Core]System.Linq.Expressions.Expression::Equal(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.Expression) L_004c: ldc.i4.1 L_004d: newarr [System.Core]System.Linq.Expressions.ParameterExpression L_0052: stloc.1 L_0053: ldloc.1 L_0054: ldc.i4.0 L_0055: ldloc.0 L_0056: stelem.ref L_0057: ldloc.1 L_0058: call class [System.Core]System.Linq.Expressions.Expression`1<!!0> [System.Core]System.Linq.Expressions.Expression::Lambda<class [mscorlib]System.Func`2<class LinqToSqlTest.MyEntity, bool>>(class [System.Core]System.Linq.Expressions.Expression, class [System.Core]System.Linq.Expressions.ParameterExpression[]) L_005d: call class [System.Core]System.Linq.IQueryable`1<!!0> [System.Core]System.Linq.Queryable::Where<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>, class [System.Core]System.Linq.Expressions.Expression`1<class [mscorlib]System.Func`2<!!0, bool>>) L_0062: call !!0 [System.Core]System.Linq.Queryable::FirstOrDefault<class LinqToSqlTest.MyEntity>(class [System.Core]System.Linq.IQueryable`1<!!0>) L_0067: pop L_0068: ret } </code></pre> <p>And the difference:</p> <p><img src="https://i.stack.imgur.com/jlQqe.jpg" alt="WithOperatorAndSelect and WithOperatorAndWhere difference"></p> <p>The only difference (in addition to that WithOperatorAndSelect works :) ) is the Select statement in MSIL.</p> <p>So I <em>guess</em> it's not an == operator / Equals problem. But I don't know.</p>

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