Value of this property will be correct after execution of the PostData method.
Value of this property will be changed only when the uploading process will be in state StatusCode.Sending.
Please see example here: PostData.
Target Platforms: .NET 9; .NET 8; .NET 7; .NET 6; .NET Framework 4.8, 4.7, 4.6, 4.5, 4.0, 3.5