C# texture2d how to convert to cpu image

You need to create a staging texture, that you can then access through cpu.

To do so, I assume that you already have an existing SharpDX texture :

    public static StagingTexture2d FromTexture(Texture2D texture)
    {
        if (texture == null)
            throw new ArgumentNullException("texture");

        //Get description, and swap a few flags around (make it readable, non bindable and staging usage)
        Texture2DDescription description = texture.Description;
        description.BindFlags = BindFlags.None;
        description.CpuAccessFlags = CpuAccessFlags.Read;
        description.Usage = ResourceUsage.Staging;

        return new StagingTexture2d(texture.Device, description);
    }

This new texture will allow read operations.

Next you need to copy your GPU texture into the staging Texture using the device context :

deviceContext.CopyResource(gpuTexture, stagingTexture);

Once this is done, you can map the staging texture to access it's content on the CPU :

DataStream dataStream;
DataBox dataBox = deviceContext.MapSubresource(stagingTexture,0, MapMode.Read, MapFlags.None, out dataStream); 

//Use either datastream to read data, or dataBox.DataPointer
//generally it's good to make a copy of that data immediately and unmap asap

//Very important, unmap once you done
deviceContext.UnmapSubresource(stagingTexture, 0);