Is there a standard class to represent a “range” in .net?

That's correct, before 2020, there was no built-in class in C# or the BCL for ranges. However, there is TimeSpan in the BCL for representing time spans, which you can compose additionally with a DateTime to represent a range of times.

AFAIK there is no such thing in .NET. Would be interesting though, to come up with a generic implementation.

Building a generic BCL quality range type is a lot of work, but it might look something like this:

public enum RangeBoundaryType
{
    Inclusive = 0,
    Exclusive
}

public struct Range<T> : IComparable<Range<T>>, IEquatable<Range<T>>
    where T : struct, IComparable<T>
{
    public Range(T min, T max) : 
        this(min, RangeBoundaryType.Inclusive, 
            max, RangeBoundaryType.Inclusive)
    {
    }

    public Range(T min, RangeBoundaryType minBoundary,
        T max, RangeBoundaryType maxBoundary)
    {
        this.Min = min;
        this.Max = max;
        this.MinBoundary = minBoundary;
        this.MaxBoundary = maxBoundary;
    }

    public T Min { get; private set; }
    public T Max { get; private set; }
    public RangeBoundaryType MinBoundary { get; private set; }
    public RangeBoundaryType MaxBoundary { get; private set; }

    public bool Contains(Range<T> other)
    {
        // TODO
    }

    public bool OverlapsWith(Range<T> other)
    {
        // TODO
    }

    public override string ToString()
    {
        return string.Format("Min: {0} {1}, Max: {2} {3}",
            this.Min, this.MinBoundary, this.Max, this.MaxBoundary);
    }

    public override int GetHashCode()
    {
        return this.Min.GetHashCode() << 256 ^ this.Max.GetHashCode();
    }

    public bool Equals(Range<T> other)
    {
        return
            this.Min.CompareTo(other.Min) == 0 &&
            this.Max.CompareTo(other.Max) == 0 &&
            this.MinBoundary == other.MinBoundary &&
            this.MaxBoundary == other.MaxBoundary;
    }

    public static bool operator ==(Range<T> left, Range<T> right)
    {
        return left.Equals(right);
    }

    public static bool operator !=(Range<T> left, Range<T> right)
    {
        return !left.Equals(right);
    }

    public int CompareTo(Range<T> other)
    {
        if (this.Min.CompareTo(other.Min) != 0)
        {
            return this.Min.CompareTo(other.Min);
        }

        if (this.Max.CompareTo(other.Max) != 0)
        {
            this.Max.CompareTo(other.Max);
        }

        if (this.MinBoundary != other.MinBoundary)
        {
            return this.MinBoundary.CompareTo(other.Min);
        }

        if (this.MaxBoundary != other.MaxBoundary)
        {
            return this.MaxBoundary.CompareTo(other.MaxBoundary);
        }

        return 0;
    }
}