单个泛型上的可比较和可空约束

Question

I want to write an extension method which compares a value against two other values and determines if the value is within the other two. This works for that purpose

public static bool IsWithin<T>(this T value, T min, T max) where T : IComparable<T>
{
    return (bool)(value.CompareTo(min) > 0 && value.CompareTo(max) < 0);
}

I would also like to extend this method to work on Nullable<T> . I would like either boundary condition to be ignored if the value is null. I have tried putting an additional constraint on T, but this is not supported. The following code doesn't compile, but is what I want to achieve.

public static bool IsWithinInclusive<T>(this T value, Nullable<T> min, Nullable<T> max) where T : IComparable<T>
{
    return (bool)(
        (min.HasValue ? value.CompareTo(min) >= 0 : true) && 
        (max.HasValue ? value.CompareTo(max) <= 0 : true));
}

Is it possible write a single method which achieves this?

Answer 1

You need to constrain T to be a struct to match the definition of Nullable<T> :

public static bool IsWithinInclusive<T>(this T value, Nullable<T> min, Nullable<T> max) where T : struct, IComparable<T>
{
    return 
        (min.HasValue ? value.CompareTo(min.Value) >= 0 : true) && 
        (max.HasValue ? value.CompareTo(max.Value) <= 0 : true);
}

You also need to use .Value to get the T from the Nullable<T> .

Answer 2

As SLaks answered, the reason Nullable<T> (or just T? ) isn't accepted is because that requires T to be a non-nullable value type. You cannot use a single method that covers value and reference types alike: the value types you want to support can't represent null , and reference types can't support T? , so you can't avoid having two separate methods.

However, you can use the exact same function body twice, to make it obvious to a reader that the two overloads do the same thing:

public static bool IsBetween<T>(this T value, T min, T max) where T : IComparable<T>
{
    return (min == null || value.CompareTo((T)min) >= 0)
        && (max == null || value.CompareTo((T)max) <= 0);
}

public static bool IsBetween<T>(this T value, T? min, T? max) where T : struct, IComparable<T>
{
    return (min == null || value.CompareTo((T)min) >= 0)
        && (max == null || value.CompareTo((T)max) <= 0);
}