using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Text;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            SourceText origSourceText = SourceText.From("class Test { }");
            SyntaxTree origSyntaxTree = SyntaxFactory.ParseSyntaxTree(origSourceText);

            TextChange textChange = new TextChange(new TextSpan(12, 0), "public void Test() {}");
            SourceText changedSourceText = origSourceText.WithChanges(textChange);
            SyntaxTree changedSyntaxTree = origSyntaxTree.WithChangedText(changedSourceText);

            string src = changedSyntaxTree.GetCompilationUnitRoot().NormalizeWhitespace().ToString();
            /*src
            class Test
            {
                public void Test()
                {
                }
            }
            */
        }
    }
}