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POCompare with string values using IQueryable<T> and Expressions
 

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  1. POCompare with string values using IQueryable<T> and Expressions
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    <p>Sorry about the title but I can't really think of a quick way of saying what I want - could someone please change it to a more appropriate one if you think of one?</p> <p>I'm trying to turn the following function call into an expression query.</p> <pre><code>List.Compare("propretyName", "&gt;1000"); public static IQueryable&lt;T&gt; Compare&lt;T&gt;(this IQueryable&lt;T&gt; source, string propertyName, string value) { Type type = typeof(T); ParameterExpression parameter = Expression.Parameter(type, "param"); MemberExpression memberAccess = Expression.MakeMemberAccess(parameter, type.GetProperty(propertyName)); string comparisonType = value.Substring(0, 1); value = value.Length &gt; 0 ? value.Substring(1) : "0"; decimal tmpvalue; decimal? result = decimal.TryParse(value, out tmpvalue) ? tmpvalue : (decimal?)null; ConstantExpression constant = Expression.Constant(result, typeof(decimal?)); BinaryExpression comparisonExpression = Expression.GreaterThan(memberAccess, constant); switch (comparisonType) { case "&gt;": comparisonExpression = Expression.GreaterThan(memberAccess, constant); break; case "&lt;": comparisonExpression = Expression.LessThan(memberAccess, constant); break; case "=": comparisonExpression = Expression.Equal(memberAccess, constant); break; } Expression&lt;Func&lt;T, bool&gt;&gt; lambda = Expression.Lambda&lt;Func&lt;T, bool&gt;&gt;(comparisonExpression, parameter); return source.Where(lambda); } </code></pre> <p>The above is the method I wrote to make that call.</p> <p>The lambda at the bottom appears to be correct: <code>{param =&gt; (param.propretyName &gt; 1000)}</code></p> <p>However, it's not working and I think it's because the particular proprety I'm working on is a <code>decimal?</code> so it should be <code>{param =&gt; (param.propertyName.Value &gt; 1000)}</code>.</p> <p>Could anybody help me out to use the Value rather than. There's just something that's escaping me here.</p> <p>I can't use the <code>Where(string)</code> method as I'm using Entity-Framework.</p> <h1>Answer found</h1> <pre><code>public static IQueryable&lt;T&gt; Compare&lt;T&gt;(this IQueryable&lt;T&gt; source, string propertyName, string value) { Type type = typeof(T); ParameterExpression parameter = Expression.Parameter(type, "param"); MemberExpression memberAccess = Expression.MakeMemberAccess(parameter, type.GetProperty(propertyName)); //This is the added methods that results in the proper output PropertyInfo valProp = typeof(Nullable&lt;decimal&gt;).GetProperty("Value"); memberAccess = Expression.MakeMemberAccess(memberAccess, valProp); string comparisonType = value.Substring(0, 1); value = value.Length &gt; 0 ? value.Substring(1) : "0"; decimal tmpvalue; decimal? result = decimal.TryParse(value, out tmpvalue) ? tmpvalue : (decimal?)null; ConstantExpression constant = Expression.Constant(tmpvalue); BinaryExpression comparisonExpression = Expression.GreaterThan(memberAccess, constant); switch (comparisonType) { case "&gt;": comparisonExpression = Expression.GreaterThan(memberAccess, constant); break; case "&lt;": comparisonExpression = Expression.LessThan(memberAccess, constant); break; case "=": comparisonExpression = Expression.Equal(memberAccess, constant); break; } Expression&lt;Func&lt;T, bool&gt;&gt; lambda = Expression.Lambda&lt;Func&lt;T, bool&gt;&gt;(comparisonExpression, parameter); return source.Where(lambda); } </code></pre>
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    1. COWhat is the error that you are getting? It works fine for the `decimal?`property that I tested.
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    2. COSpecifically it's related to EF. EF needs it to be `propertyName.Value`.
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    3. COIs the passed property decimal or nullable decimal ?
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      1. USHomam
 

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