这里有一个Nullable Enum返回值的工作示例:

public static TEnum? ParseOptional<TEnum>(this string value) where TEnum : struct
{
    return value == null ? (TEnum?)null : (TEnum) Enum.Parse(typeof(TEnum), value);
}

将类约束作为第一个约束添加到泛型类型中。

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

三个选项:

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.

上述2个答案的另一种选择。如果将返回类型更改为object，则可以返回null，同时转换非null返回。

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

return default(T);

因为IThing是接口不可能使用null。因此，必须使用default(T)来确定实际类型T的默认值，该类型T是在调用函数之前定义的。

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;}}