ImageSharp is fully managed and cross-platform. It reads and writes TGA (8, 16, 24 and 32 bits, raw or RLE) and converts to any other format it supports. Free for open source and small companies, commercial license otherwise (Six Labors Split License).
Magick.NET wraps ImageMagick: reads and writes TGA and 100+ other formats. Apache 2.0, ships native binaries.
Pfim only decodes TGA and DDS, with no dependencies and very little overhead. Good for game tools and texture viewers that hand pixels to WPF, GDI+ or a GPU.
ImageSharpImageSharp is a managed, cross-platform .NET imaging library. How to read and write TGA files in C#, choose bits per pixel, enable RLE and convert TGA to PNG.
Magick.NETMagick.NET is ImageMagick for .NET. How to read and write TGA files in C#, convert TGA to PNG, write RLE-compressed TGA and read the size without decoding.
PfimPfim is a small, fast .NET decoder for TGA and DDS images. How to decode a TGA in C#, read the header and convert the pixels to PNG or a WPF bitmap.