如何在方法返回中使用具有两种类型的通用接口

Question

I need to create an interface with 2 type and use it as a method return value.

public interface StringLong<T1,T2>
where T1 : string
where T2 : long
{}

StringLong<T1,T2> method StringLong<T1,T2>()

Answer 1

It makes no sense to define an interface with two generic types that you constrain to just string and long .

It sounds like you just want a tuple:

(string, long) MyMethod()
{
    return ("Hello", 42L);
}

You can even name the return values:

(string message, long meaningOfLife) MyMethod()
{
    return ("Hello", 42L);
}

Then you can write:

var result = MyMethod();
Console.WriteLine(result.message);
Console.WriteLine(result.meaningOfLife);

Answer 2

I think is the functionality you are trying to achieve (from the comments). Since the return might be of either string or long there common ancestor is object .

Once you have the value you can use pattern matching to cast the result into the appropriate type:

static class Program
{
    static void Main(string[] args)
    {
        var obj = MethodReturnsStringOrLong(1722);
        switch (obj)
        {
            case string str:
                Console.WriteLine($"String is {str}");
                break;
            case long lng:
                Console.WriteLine($"Long is {lng}");
                break;
            default:
                throw new NotSupportedException();
        }
    }

    public static object MethodReturnsStringOrLong(int input)
    {
        if (input % 2 == 0)
        {
            return 1928374028203384L;
        }
        else
        {
            return "ASIDJMFHSOASKSJHD";
        }
    }
}

---

An alternative is the create your own common ancestor, like the class Value below that might contains either a long and/or a string .

public class Value
{
    public Value(long longValue)
    {
        LongValue = longValue;            
    }
    public Value(string stringValue)
    {
        StringValue = stringValue;
    }

    public long? LongValue { get; }
    public string StringValue { get; }
}


static class Program
{
    static void Main(string[] args)
    {
        var obj = MethodReturnsStringOrLong(1722);
        if (obj.LongValue.HasValue)
        {
            Console.WriteLine($"Long is {obj.LongValue.Value}");
        }
        if (!string.IsNullOrEmpty(obj.StringValue))
        {
            Console.WriteLine($"String is {obj.StringValue}");
        }
    }

    public static Value MethodReturnsStringOrLong(int input)
    {
        if (input % 2 == 0)
        {
            return new Value(1928374028203384L);
        }
        else
        {
            return new Value("ASIDJMFHSOASKSJHD");
        }
    }

}