如何比较相同泛型类型的两个值？

Question

Following code won't compile:

class Test<T> where T : class, IComparable
{
    public bool IsGreater(T t1, T t2)
    {
        return t1 > t2; // Cannot apply operator '>' to operands of type 'T' and 'T'
    }
}

How I can make it work? I want it to work for T = int , double , DateTime , etc.

Answer 1

The IComparable interface that you have used here as the type constraint gives you a CompareTo method. So you can do something like this:

public bool IsGreater(T t1, T t2)
{
    return t1.CompareTo(t2) > 0;
}

Answer 2

While currently you can't do exactly that (and you need to use solution by @DavidG ) there is a preview feature you can enable called generic math which is based on static abstract interface members and ships with interface specially for that purpose:

[RequiresPreviewFeatures]
public interface IComparisonOperators<TSelf, TOther> : IComparable, IComparable<TOther>, IEqualityOperators<TSelf, TOther>, IEquatable<TOther> where TSelf : IComparisonOperators<TSelf, TOther>
{
    static bool operator <(TSelf left, TOther right);

    static bool operator <=(TSelf left, TOther right);

    static bool operator >(TSelf left, TOther right);

    static bool operator >=(TSelf left, TOther right);
}

So in future (or right now if you want to be an early adopter) you will be able to do (if no major overhaul comes for this preview):

class Test<T> where T : IComparisonOperators<T, T>
{
    public bool IsGreater(T t1, T t2)
    {
        return t1 > t2; 
    }
}

Note that for Test to support int , double , DateTime , etc. you need to remove class constraint (with or without this preview feature).

Answer 3

You can use this to compare two values of same generic type.

public bool IsGreater(T t1, T t2)
{
    return t1.CompareTo(t2) > 0;
}

Answer 4

Just let .NET compare for you:

public bool IsGreater(T t1, T t2) => Comparer<T>.Default.Compare(t1, t2) > 0; 

Note, that you don't have to check for null