using System;

using System.Linq;

using System.Reflection;

​

namespace Example

{

    public interface IExternalInterface

    {

        string Text { get; set; }

    }

    public class ExternalClass : IExternalInterface

    {

        public string Text { get; set; }

    }

    public class ClassUnderTest

    {

        public void MethodUnderTest(IExternalInterface injected)

        {

            Console.WriteLine("The injected class has a Text value of {0}", injected.Text);

        }

    }

​

    public static class ExtensionMethods

    {

        public static void CallWithMockedParameter(this object objectUnderTest, string methodName, object valuesForMockedParameter)

        {

            var typeUnderTest = objectUnderTest.GetType();

            var methodUnderTest = typeUnderTest.GetMethod(methodName, BindingFlags.Public | BindingFlags.Instance);

            var parameterUnderTest = methodUnderTest.GetParameters()[0];

            var parameterType = parameterUnderTest.ParameterType;

            var typeToPass = parameterType.Assembly.GetTypes().Where( a => parameterType.IsAssignableFrom(a) && a.IsClass).Single();

            var objectToPass = Activator.CreateInstance(typeToPass);

            foreach (var propertyToCopy in valuesForMockedParameter.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance))

            {

                typeToPass.GetProperty(propertyToCopy.Name).SetValue(objectToPass, propertyToCopy.GetValue(valuesForMockedParameter));

            }

            methodUnderTest.Invoke(objectUnderTest, new[] { objectToPass });

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            //Normal execution

            ClassUnderTest o1 = new ClassUnderTest();

            IExternalInterface a1 = new ExternalClass { Text = "Normal" };

            o1.MethodUnderTest(a1);

            //Mocked

            var o2 = new ClassUnderTest();

            var a2 = new { Text = "Mocked" };

            o2.CallWithMockedParameter(nameof(o2.MethodUnderTest), a2);

        }

    }

}

​