How can I return NULL from a generic method in C#?

Edoode_6399 0
Viewed 220996

I have a generic method with this (dummy) code (yes I'm aware IList has predicates, but my code is not using IList but some other collection, anyway this is irrelevant for the question...)

    static T FindThing<T>(IList collection, int id) where T : IThing, new()
    {
        foreach (T thing in collection)
        {
            if (thing.Id == id)
                return thing;
        }
        return null;  // ERROR: Cannot convert null to type parameter 'T' because it could be a value type. Consider using 'default(T)' instead.
    }

This gives me a build error

"Cannot convert null to type parameter 'T' because it could be a value type. Consider using 'default(T)' instead."

Can I avoid this error?

generics c#
13 Answers

Three options:

  • Return default (or default(T) for older versions of C#) which means you'll return null if T is a reference type (or a nullable value type), 0 for int, '\0' for char, etc. (Default values table (C# Reference))
  • If you're happy to restrict T to be a reference type with the where T : class constraint and then return null as normal
  • If you're happy to restrict T to be a non-nullable value type with the where T : struct constraint, then again you can return null as normal from a method with a return value of T? - note that this is not returning a null reference, but the null value of the nullable value type.
user_226560 
return default(T);
Ricardo_Villamil_193140

You can just adjust your constraints:

where T : class

Then returning null is allowed.

TheSoftwareJedi_189410

Add the class constraint as the first constraint to your generic type.

static T FindThing<T>(IList collection, int id) where T : class, IThing, new()
Min_144610

Your other option would be to to add this to the end of your declaration:

    where T : class
    where T: IList

That way it will allow you to return null.

BFree_158610

For completeness sake, it's good to know you could also do this:

return default;

It returns the same as return default(T);

user_14397480

Because of IThing is interface is not possible to use null. So you have to use default(T) to determine default value for actual type T which is defined before function is called.

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
        
        IThing x = new List<Thing>().FindThing(1);
        
    }

}

public static class Ext {
    public static T FindThing<T>(this IList<T> collection, int id) where T : IThing, new()
    {
        foreach (T thing in collection)
        {
            if (thing.Id == id) return (T)thing;
        }
    
        //return null; //not work
        //return (T)null; //not work
        //return null as T; //not work
        return default(T); //work
    }
}

public interface IThing { int Id {get; set;} }
public class Thing : IThing { public int Id {get;set;}}
user_10711650

Take the recommendation of the error... and either user default(T) or new T.

You will have to add in a comparison in your code to ensure that it was a valid match if you go that route.

Otherwise, potentially consider an output parameter for "match found".

Mitchel_Sellers_132790

Another alternative to 2 answers presented above. If you change your return type to object, you can return null, while at the same time cast the non-null return.

static object FindThing<T>(IList collection, int id)
{
    foreach T thing in collecion
    {
        if (thing.Id == id)
            return (T) thing;
    }
    return null;  // allowed now
}
user_8685320
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