How can I use reflection to convert from int to decimal?

Answer 1

You will have to do the conversion yourself, as the compiler handles the cast in a non-reflection environment. As reflection code basically is evaluating types and objects like the compiler does, you will have to look for a method named op_implicit with the needed parameters (in your case Int32) on your object and invoke it. After that, you can invoke the property accessor. A possible way would be the following:

//search for an implicit cast operator on the target type
MethodInfo[] methods = targetType.GetMethods();
foreach(MethodInfo method = source.GetType().GetMethod("op_Implicit"))
{
  if (method.Name == "op_Implicit")
  {
    ParameterInfo[] parameters = method.GetParameters();
    if (parameters.Length == 1 && parameters[0].ParameterType == value.GetType())
    {
      value = method.Invoke(obj,new object[]{value});
      break;
    }
  }
}

Answer 2

Why would an implicit type conversion that works fine elsewhere fail with reflection?

Because there is no conversion there. Implicit conversion doesn't mean that it happens automatically, when you use it in code the compiler adds the code for it.

If you want to use it in reflection, you have to do the same, i.e. find the static method that does the conversion from one type to the other, and call it.

Answer 3

The runtime doesn't know about implicit conversions.

You can call op_Implicit or another conversion method through reflection, but that way you'll only get the specific conversion semantics that you implement. If you're using C# 4.0, I'd recommend using the "dynamic" type here, since it will implement the C# conversion semantics automatically.

Answer 4

Other answers have already covered why the implicit conversion isn't working. If you need your conversion to be implicit, then use one of the other answers. In case you don't actually need the conversion to be implicit, here is a simpler option:

class Test
{
  public decimal? Val { get; set; }
}

class Program
{
  static void Main(string[] args)
  {
    object o = new Test();
    object source = 5;
    var setMethod = typeof(Test).GetProperty("Val").GetSetMethod();
    // Just do the cast explicitly
    setMethod.Invoke(o, new object[] { (decimal?)(int)source });
  }
}

Note that if you're missing the (decimal?) cast, you get the error that the original question quoted. If you're missing the (int) cast, you get this error:

Unhandled Exception: System.InvalidCastException: Specified cast is not valid.
   at Program.Main(String[] args) in ...\ConsoleApplication1\Program.cs:line 14