Partial evaluation of expressions

It looks like it is not possible to specify this at compile time, but one can compile and evaluate the expression parts at runtime using Expression.Lambda<>(...).Compile()() (see documentation).

This results in the following implementation (no error checking for sake of simplicity):

public static void HandleMethodCall<T>(Expression<Func<T, object>> expr)
{
    // Extract method call
    var methodCall = (MethodCallExpression)expr.Body;

    // Run through expected arguments and evaluate the supplied ones
    var expectedArguments = methodCall.Method.GetParameters();
    for(int i = 0; i < expectedArguments.Length; ++i)
    {
        Console.Write(expectedArguments[i].Name + ": ");

        // Since we do not know the argument type in advance, we have to use Func<object>
        var argumentEvaluationExpression = Expression.Lambda<Func<object>>(Expression.Convert(methodCall.Arguments[i], typeof(object)));
        object argumentValue = argumentEvaluationExpression.Compile()();

        Console.WriteLine(argumentValue);
    }
}

Calling this function with

HandleMethodCall<Test>(test => test.SomeMethod("string".Length, "1" + " 2 " + (1 + 2).ToString()));

outputs the expected result

a: 6
b: 1 2 3

However, I am not sure whether this will really work under all conditions. In addition, I expect this to be quite slow due to the Compile() step.

Thanks to @RyanWilson for the helpful comments, which allowed me to figure out this solution!