Expression Trees in .NET 4.0: Expression.Call fails to find method "get_Item" in type List<T>

Question

Hi there,

I'm stuck with the problem below and wondering is someone out there will be able to help. I have added comments to the code to make it self-explanatory but let me know if you need more info or if the problem is unclear.

Thanks a lot in advance!

Edit: I've been asked to summarize the problem in text, so here it goes: under the circumstances described in the code below, Expression.Call(...) throws the following exception: "No method 'get_Item' exists on type 'System.Collections.Generic.List`1[System.Double]'"

I believe the method does exist in the type, as shown here:

List<double> sampleList = new List<double>();

Console.WriteLine(sampleList.GetType().GetMethod("get_Item") == null); // False

I've also made the title a bit more descriptive; sorry if the initial question was not clear.

public class ExpressionExample
{
    public void Main()
    {
        Expression<Func<List<double>, double>> component = u => u[0];
        Console.WriteLine(component.Body.NodeType); // Prints out "Call"
        Console.WriteLine(component.Body); // Prints out "u.get_Item(0)"

        MethodCallExpression copyGetComponent = CopyCallExpression(component.Body as MethodCallExpression);
    }

    public MethodCallExpression CopyCallExpression(MethodCallExpression callExpression)
    {
        if (callExpression == null)
            return null;

        // Some tests
        Console.WriteLine(callExpression.Method.Name); // "get_Item"
        List<double> sampleList = new List<double>();
        Console.WriteLine(sampleList.GetType().GetProperty("get_Item") == null); // True
        Console.WriteLine(sampleList.GetType().GetProperty("Item") == null); // False
        Console.WriteLine(sampleList.GetType().GetMethod("get_Item") == null); // False (1)
        Console.WriteLine(sampleList.GetType().GetMethod("Item") == null); // True            
        Console.WriteLine(sampleList.GetType().FullName == callExpression.Method.DeclaringType.FullName); // True! (2)

        // However...
        Type[] argTypes = (from argument in callExpression.Arguments select argument.Type).ToArray();
        // Next line throws an exception: No method 'get_Item' exists on type 'System.Collections.Generic.List`1[System.Double]'
        return Expression.Call(callExpression.Method.DeclaringType, callExpression.Method.Name, argTypes, callExpression.Arguments.ToArray());

        // How does this come together with items (1) and (2) above?
    }
}

Edit: I think I've found a workaround that solves my immediate problem; posting it in case someone else out there is struggling with this:

public class ExpressionExample
{
    public void Main()
    {
        Expression<Func<List<double>, double>> invokeProp = u => u[0];

        Console.WriteLine(invokeProp); // u => u.get_Item(0)
        Console.WriteLine(invokeProp.Body); // u.get_Item(0)
        Console.WriteLine(invokeProp.Body.NodeType); // Call

        Expression copyGetComponent = CopyCallExpression(invokeProp.Body as MethodCallExpression);

        LambdaExpression copyInvokeProp = Expression.Lambda(copyGetComponent, invokeProp.Parameters);

        Console.WriteLine(copyInvokeProp); // u => u.Item[0]
        Console.WriteLine(copyInvokeProp.Body); // u.Item[0]
        Console.WriteLine(copyInvokeProp.Body.NodeType); // Index

        // Technically different expressions, but I suppose
        // they should be "functionally equal" though
    }

    public Expression CopyCallExpression(MethodCallExpression callExpression)
    {
        if (callExpression == null)
            return null;

        MethodInfo info = callExpression.Method;

        if (info.Name == "get_Item")
        {
            PropertyInfo propInfo = typeof(List<double>).GetProperty("Item");
            return Expression.MakeIndex(callExpression.Object, propInfo, callExpression.Arguments);
        }

        if (info.IsStatic)
            return Expression.Call(info, callExpression.Arguments);

        Type[] argTypes = (from argument in callExpression.Arguments select argument.Type).ToArray();
        return Expression.Call(info.DeclaringType, info.Name, argTypes, callExpression.Arguments.ToArray());
    }

Answer 1

The overload you use is for static methods only (see documentation: http://msdn.microsoft.com/en-us/library/bb351107.aspx). And "get_Item" is clearly not a static method. So, you need to use a different method overload. Try this, for example:

return Expression.Call(callExpression.Object, callExpression.Method, callExpression.Arguments.ToArray());