ColumnReference - easy way to convert between Excel column index and column letter

ColumnReference.cs

internal struct ColumnReference
{
  readonly uint _ColumnIndex;
  const int AsciiTrim = 'A' - 1;
  const int LastChar = 'Z' - AsciiTrim;

  ColumnReference(uint columnIndex)
  {
    _ColumnIndex = columnIndex;
  }

  ColumnReference(string columnName)
  {
    columnName = columnName.ToUpperInvariant();

    _ColumnIndex =
      (uint)columnName
        .Reverse()
        .Select(ch => ch - AsciiTrim)
        .Select((ch, i) => ch * Math.Pow(LastChar, i))
        .Sum()
        - 1; //make zero-index based
  }

  public string ColumnName
  {
    get
    {
      var colNumber = _ColumnIndex + 1; //non-zero-based          
      var output = new Stack<char>();

      //var index = 1;
      while (colNumber > 0)
      {
        var current = colNumber % LastChar;

        if (current == 0) current = LastChar;

        output.Push((char)(current + AsciiTrim));
        colNumber = (colNumber - current) / LastChar;
      }
      return string.Concat(output);
    }
  }

  public override string ToString() => ColumnName;

  public static implicit operator uint(ColumnReference r) => r._ColumnIndex;
  public static implicit operator int(ColumnReference r) => (int)r._ColumnIndex;
  public static implicit operator string(ColumnReference r) => r.ColumnName;

  public static explicit operator ColumnReference(uint index) => new ColumnReference(index);
  public static explicit operator ColumnReference(int index) => new ColumnReference((uint)index);
  public static explicit operator ColumnReference(string columnName) => new ColumnReference(columnName);
}

ColumnReferenceTests.cs

[Theory]
[InlineData("A", 0)]
[InlineData("Z", 25)]
[InlineData("AA", 26)]
[InlineData("AZ", 51)]
[InlineData("ABC", 730)]
public void TestColRefs(string colName, int expectedIndex)
{
  Assert.Equal((ColumnReference)colName, expectedIndex);

  Assert.Equal((ColumnReference)expectedIndex, colName);
}