具有可为空类型的泛型约束

Question

I've got these very clearly defined methods that internally all call InternalGreaterThan() but can't find a way - without removing the T constraint - of making this work for long? and DataTime?

#nullable enable
public static bool GreaterThan(long? @this, long? value)
{
    return InternalGreaterThan(@this, value);
}

public static bool GreaterThan(DateTime? @this, DateTime? value)
{
    return InternalGreaterThan(@this, value);
}

public static bool GreaterThan(Version? @this, Version? value)
{
    return InternalGreaterThan(@this, value);
}

private static bool InternalGreaterThan<T>(T @this, T value) where T : IComparable<T>?
{
    return @this != null && value != null && Comparer<T>.Default.Compare(@this, value) > 0;
}

What are my options here without having to remove the T constraint (although altering the constraint is of course fine).

Answer 1

You need two separate overloads. One for T and one for T? . Eg:

private static bool InternalGreaterThan<T>(T @this, T value) where T : IComparable<T>
{
    return Comparer<T>.Default.Compare(@this, value) > 0;
}

private static bool InternalGreaterThan<T>(T? @this, T? value) where T : struct, IComparable<T>
{
    return @this.HasValue && value.HasValue && InternalGreaterThan(@this.Value, value.Value);
}

Answer 2

You cannot do that. Assuming, you can. Than it is possible to make a call like this:

DateTime? dt = null;
var result = dt.GreaterThan(null);

The system is unable to detect the argument's type. Which version of the method the system should take?