Expression, Constant List, Compiler generated calss

Question

I have this simple code:

public void MyWhere( Expression<Func<T, bool>> predicate)
{

}

List<string> Indexes2 = new List<string>();
Indexes2.Add("abc");
MyWhere(a=>Index2.Contains(a.a1));

While parsing the expression, that Index2 appears as a ConstantExpression. Then similar to many examples on this site and elsewhere, I have this method for parsing value of ConatantExpression:

private static object ConstantValue(ConstantExpression member)
{
    // source: http://stackoverflow.com/a/2616980/291955
    var objectMember = Expression.Convert(member, typeof(object));
    var getterLambda = Expression.Lambda<Func<object>>(objectMember);
    var getter = getterLambda.Compile();
    return getter();
}

Problem is in return type of this method, type of return value is:

{Name = "<>c__DisplayClass38_0" FullName = "S_Common.A_Dictionary`2+<>c__DisplayClass38_0[[S_Common.StringIndex, S_Common, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null],[DummyTestApp.test, DummyTestApp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]"}

In QuickWatch it is possible to find the underlying List, but almost no way to refer it in code.

Answer 1

When you "close" a local variable, an hidden class is generated. What you see in the ConstantExpression is a reference to an instance of this hidden class.

This:

public void MyWhere<T>(Expression<Func<T, bool>> predicate)
{
}

public void M() 
{
    List<string> Indexes2 = new List<string>();
    Indexes2.Add("abc");
    MyWhere<String>(a => Indexes2.Contains(a));
}

is compiled to

[CompilerGenerated]
private sealed class <>c__DisplayClass1_0
{
    public List<string> Indexes2;
}

public void MyWhere<T>(Expression<Func<T, bool>> predicate)
{
}

public void M()
{
    <>c__DisplayClass1_0 <>c__DisplayClass1_ = new <>c__DisplayClass1_0();
    <>c__DisplayClass1_.Indexes2 = new List<string>();
    <>c__DisplayClass1_.Indexes2.Add("abc");
    ParameterExpression parameterExpression = Expression.Parameter(typeof(string), "a");
    MemberExpression instance = Expression.Field(Expression.Constant(<>c__DisplayClass1_, typeof(<>c__DisplayClass1_0)), FieldInfo.GetFieldFromHandle((RuntimeFieldHandle)/*OpCode not supported: LdMemberToken*/));
    MethodInfo method = (MethodInfo)MethodBase.GetMethodFromHandle((RuntimeMethodHandle)/*OpCode not supported: LdMemberToken*/, typeof(List<string>).TypeHandle);
    Expression[] array = new Expression[1];
    array[0] = parameterExpression;
    MethodCallExpression body = Expression.Call(instance, method, array);
    ParameterExpression[] array2 = new ParameterExpression[1];
    array2[0] = parameterExpression;
    MyWhere(Expression.Lambda<Func<string, bool>>(body, array2));
}

(see sharplab)

The interesting parts are the private sealed class <>c__DisplayClass1_0 and the Expression.Constant(<>c__DisplayClass1_, typeof(<>c__DisplayClass1_0)).

This hidden class is hidden. You can interact with it only through reflection.

Your problem isn't really soluble in an easy way. For the specific example given:

public static void MyWhere<T>(Expression<Func<T, bool>> predicate)
{
    var body = predicate.Body;

    // .Contains(...)
    var contains = body as MethodCallExpression;

    // Indexes2
    var field = contains.Object;

    // Need boxing only for value types
    var boxIfNecessary = field.Type.IsValueType ? (Expression)Expression.Convert(field, typeof(object)) : field;
    var lambda = Expression.Lambda<Func<object>>(boxIfNecessary);
    var compiled = lambda.Compile();

    // Indexes of type List<string>()
    var value = compiled();
}

For example just this:

MyWhere<string>(a => Enumerable.Contains(Indexes2, a));

will break the code I gave.