How do you check if a directory exists c#?

For checking if a file exists I use : if (!File.Exists("my file")) but how do I do that for a folder or directory?

using System.IO;

///class blah blah
// and method blah, blah


//check if directory doesn't exit
if(!Directory.Exists(directoryPath))
{	
    //if it doesn't, create it
    Directory.CreateDirectory(directoryPath);

}

You can now do:

AssetDatabase.IsValidFolder(string path);