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POUsing expression trees to construct an object with unknown members
 

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  1. POUsing expression trees to construct an object with unknown members
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    <p>I'm trying to create a generic function that will take 2 instances of a struct and create a new instance using the values of the passed-in instances. I'm mostly there but I'm having trouble figuring out how to build the expression tree to take the new values as parameters in the MemberInit (first time using expression trees).</p> <p>I'm trying to avoid creating garbage (so no boxing) as much as possible.</p> <p>Here's what I have so far:</p> <pre><code>private static readonly Dictionary&lt;Type, FieldInfo[]&gt; fieldInfoCache = new Dictionary&lt;Type, FieldInfo[]&gt;(); private static readonly Dictionary&lt;FieldInfo, dynamic&gt; compiledDelegates = new Dictionary&lt;FieldInfo, dynamic&gt;(); private static T Lerp&lt;T&gt;(T start, T end, float amount) where T : new() { FieldInfo[] fields; var type = typeof(T); if(!fieldInfoCache.TryGetValue(type, out fields)) { fields = typeof(T).GetFields(BindingFlags.Public | BindingFlags.Instance); fieldInfoCache.Add(type, fields); } var binds = new List&lt;MemberBinding&gt;(); foreach(var fieldInfo in fields) { dynamic getter; if(!compiledDelegates.TryGetValue(fieldInfo, out getter)) { var targetExp = Expression.Parameter(type, type.Name); var fieldExp = Expression.Field(targetExp, fieldInfo); getter = Expression.Lambda(typeof(Func&lt;,&gt;).MakeGenericType(type, fieldInfo.FieldType), fieldExp, targetExp).Compile(); compiledDelegates.Add(fieldInfo, getter); } var startVal = getter.Invoke(start); var endVal = getter.Invoke(end); //This needs to be assigned to something var newVal = fieldInfo.FieldType.IsAssignableFrom(typeof(float)) ? LerpF(startVal, endVal, amount) : Lerp(startVal, endVal, amount); var fieldParamExp = Expression.Parameter(fieldInfo.FieldType, "newVal"); var bind = Expression.Bind(fieldInfo, fieldParamExp); binds.Add(bind); } //How do I fix these two lines? var memberInit = Expression.MemberInit(Expression.New(type), binds); var result = Expression.Lambda&lt;Func&lt;T&gt;&gt;(memberInit).Compile().Invoke(); return result; } </code></pre> <p>The part that I'm stumped on is how to feed the values into those last 2 lines without causing boxing</p>
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    1. COSo, the delegate you're trying to create should call `LerpF()` for each `float` field? In that case, you need to create an expression that calls that method, not just call it directly.
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    2. COYep, that's correct, but why does that need to be done inside the expression? I guess maybe if I bundled this all into 1 expression maybe... but I was planning on having 1 set of expressions for getting the data from the fields, and 1 set for constructing the new object. Unless maybe that makes the constructing part easier?
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