You cannot get the exact code, but you can get a decompiled version of it.
The most popular (and best) tool is Reflector, but there are also other .Net decompilers (such as Dis#). You can also decompile the IL using ILDASM, which comes bundled with the .Net Framework SDK Tools.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…