Question

I have a generic class that has a method ‘Call’ on it with a generic return type and a lambda expression as a parameter. This has to be generated with reflection, and since the types are unknown, I need to create the lambda expression the long way. I have a working solution listed below, but I do not like having to get the instance out of the generic class since it is private and does some logic on a retrieve.

Class:

public class Proxy<T>
{
    public TR Call<TR>(Func<T, TR> serviceInvoke)
    private T Instance
}

Reflection Usage:

var serviceInstance = proxy.GetType().GetProperty("Instance", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(proxy, null);
var serviceMethod = type.GetMethod(rule.ServiceMethodName);
var parameters = serviceMethod.GetParameters().Select(parameterInfo => PropertyChainNavigator.GetValueForPropertyChain(((string)rule.Parameters[parameterInfo.Name]).Split(new[] { '.' }), context)).ToArray();
var thisParam = Expression.Constant(serviceInstance);
var valueParams = parameters.Select(Expression.Constant).ToList();
var call = Expression.Call(thisParam, serviceMethod, valueParams);
var func = Expression.Lambda(call, Expression.Parameter(type)).Compile();
var callMethod = proxy.GetType().GetMethods().Where(x => x.Name == "Call" && x.ReturnType != typeof(void)).First().MakeGenericMethod(serviceMethod.ReturnType);
result = callMethod.Invoke(proxy, new[] { func });

Normal Usage:

var proxy = new Proxy<ITestClass>();
proxy.Call(x => x.Method);

I have tried to change the Expression.Call to not take an instance, but that only works on static methods. Does anyone know of a way for me to create the call, turn it into an lambda and have it compile without erroring

Answer 1

If you used lambda instead of expressions, your current code would look like this:

var serviceInstance = …; // retrieve using reflection
proxy.Call(i => serviceInstance.CallMethod(parameters));

Note that the lambda takes the parameter i, but doesn’t use it. I assume you want to call the method directly on i, i.e. something like this:

proxy.Call(i => i.CallMethod(parameters));

To do that, use the value of Expression.Parameter() as thisParam:

var thisParam = Expression.Parameter(type, "i"); // the name is optional
var call = Expression.Call(thisParam, serviceMethod, valueParams);
var func = Expression.Lambda(call, thisParam).Compile();