ASP.NET C# Error: The Type must be a reference type in order to use it as parameter 'T' in the generic type or method

Question

I have a class: Constants.cs

Code:

namespace DataAccess.Utilities
{
  public class Constants
  {
    public enum ReturnCode
    {
      Fail = -1,
      Success = 0,
      Warning = 1
    }
  }
}

This is my code for directcast class

public static T DirectCast<T>(object o) where T : class
{
    T value = o as T;
    if (value == null && o != null)
    {
        throw new InvalidCastException();
    }
    return value;
}

Here is my code that get's error

code = DirectCast<Constants.ReturnCode>(int.Parse(db.GetParameterValue(command, "RESULTCODE").ToString().Trim()));

Error Message:

Error 2 The type 'DataAccess.Utilities.Constants.ReturnCode' must be a reference type in order to use it as parameter 'T' in the generic type or method 'DataAccess.DataManager.QueueingManager.DirectCast(object)'

Before I am using DirectCast from .net vb, but DirectCast is not exist in c# so I try searching and I get some codes on how to create DirectCast that has same function to vb.

Answer 1

code = DirectCast<Constants.ReturnCode>(int.Parse(db.GetParameterValue(command, "RESULTCODE").ToString().Trim()));

Should be changed to:

code = DirectCast<Constants>(int.Parse(db.GetParameterValue(command, "RESULTCODE").ToString().Trim()));

Or, do it like this: see working example:

class Program
{
    static void Main(string[] args)
    {
        var fail = Constants.DirectCast<Constants.ReturnCode>(-1);
        var success = Constants.DirectCast<Constants.ReturnCode>(0);
        var warning = Constants.DirectCast<Constants.ReturnCode>(1);
        Console.WriteLine(fail);
        Console.WriteLine(success);
        Console.WriteLine(warning);
        Console.ReadLine();
    }
}

public class Constants
{
    public enum ReturnCode
    {
        Fail = -1,
        Success = 0,
        Warning = 1
    }

    public static T DirectCast<T>(object o)
    {
        T value = (T)o;
        if (value == null && o != null)
        {
            throw new InvalidCastException();
        }
        return value;
    }
}

Answer 2

In .NET, there are value types and reference types. Classes are reference types for example, they are like pointers in C++. int are value types, enums also. In generics, only reference types can be use as type arguments .

You can remove the genericity of your method but you won't be able to know the return type. Moreover, "as" won't work with enum because it's a value type.

I don't understand why you want to cast an enum, you can parse it :

Enum.Parse(typeof(Constants.ReturnCode), db.GetParameterValue(command, "RESULTCODE").ToString().Trim()));

Answer 3

Thanks for the comments. finally my code works.

 public class Constants
 {
    public enum ReturnCode
    {
        Fail = -1,
        Success = 0,
        Warning = 1
    }
 }


 public static T DirectCast<T>(object o)
    {
        T value= (T)o;
        if (value== null && o != null)
        {
            throw new InvalidCastException();
        }
        return value;
    }


 code = Constants.DirectCast<Constants.ReturnCode>(int.Parse(db.GetParameterValue(command, "RESULTCODE").ToString().Trim()));