How do I get a Non-Nullable T in C# to use it in a function?

Question

got an error in this code.

    private void Save<T>(string file)
        where T : struct, IPixel<T>
    {
        Image<T> image = Image.LoadPixelData<T>(
            _image.Data, _image.Width, _image.Height);
        image.Save(file);
    }

CS8377 C# The type 'T' must be a non-nullable value type, along with all fields at any level of nesting, in order to use it as parameter in the generic type or method

Im using C#7.3, .Net-Framework 4.6 and wpf this code works with winforms

[EDIT] Image is from Sixlabors

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;
using System;
using System.IO;

namespace DDSReader
{
public class LoadImage
{
    private readonly Pfim.IImage _image;

    public byte[] Data
    {
        get
        {
            if (_image != null)
                return _image.Data;
            else
                return new byte[0];
        }
    }

    public LoadImage(string file)
    {
        _image = Pfim.Pfim.FromFile(file);
        Process();
    }

    public LoadImage(Stream stream)
    {
        if (stream == null)
            throw new Exception("DDSImage ctor: Stream is null");

        _image = Pfim.Dds.Create(stream, new Pfim.PfimConfig());
        Process();
    }

    public LoadImage(byte[] data)
    {
        try
        {
            if (data == null || data.Length <= 0)
                throw new Exception("DDSImage ctor: no data");

            _image = Pfim.Dds.Create(data, new Pfim.PfimConfig());
            Process();
        }
        catch { }
    }

    public void Save(string file)
    {
        try
        {
            if (_image.Format == Pfim.ImageFormat.Rgba32)
                Save<Bgra32>(file);
            else if (_image.Format == Pfim.ImageFormat.Rgb24)
                Save<Bgr24>(file);
            else
                throw new Exception("Unsupported pixel format (" + _image.Format + ")");
        }
        catch { }
    }

    private void Process()
    {
        if (_image == null)
            throw new Exception("DDSImage image creation failed");

        if (_image.Compressed)
            _image.Decompress();
    }


    private void Save<T>(string file)
        where T : struct, IPixel<T>
    {
        Image<T> image = Image.LoadPixelData()

            Image.LoadPixelData<T>(
            _image.Data, _image.Width, _image.Height);
        image.Save(file);
    }

}
}

this Program reads an Image from an other File which is in DDS-Format Im Using the SixlaborsImagesharpFramework to get an Image and to read the image the PfimFramework

Answer 1

Try using unmanaged type constraint:

private void Save<T>(string file)
        where T : unmanaged, IPixel<T>

It seems that Image<T> comes from SixLabors.ImageSharp, you can check which constraints they are using at their github page.