使用Roslyn編譯器動態處理C#字符串可以通過以下步驟實現：

1. 導入所需的命名空間：

using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis;

2. 創建SyntaxTree：

string code = "Console.WriteLine(\"Hello, world!\");";
SyntaxTree syntaxTree = SyntaxFactory.ParseSyntaxTree(code);

3. 創建編譯選項：

CSharpCompilationOptions compilationOptions = new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary);

4. 創建編譯上下文：

string assemblyName = Path.GetRandomFileName();
CSharpCompilation compilation = CSharpCompilation.Create(assemblyName)
  .WithOptions(compilationOptions)
  .AddReferences(MetadataReference.CreateFromFile(typeof(object).Assembly.Location))
  .AddSyntaxTrees(syntaxTree);

5. 編譯代碼并獲取生成的程序集：

using (var ms = new MemoryStream())
{
  EmitResult result = compilation.Emit(ms);
  if (!result.Success)
  {
    IEnumerable<Diagnostic> failures = result.Diagnostics.Where(diagnostic =>
      diagnostic.IsWarningAsError ||
      diagnostic.Severity == DiagnosticSeverity.Error);
  
    foreach (Diagnostic diagnostic in failures)
    {
      Console.Error.WriteLine("{0}: {1}", diagnostic.Id, diagnostic.GetMessage());
    }
  }
  else
  {
    ms.Seek(0, SeekOrigin.Begin);
    Assembly assembly = Assembly.Load(ms.ToArray());
    dynamic compiledCode = Activator.CreateInstance(assembly.GetType("TestProgram"));
    compiledCode.Main();
  }
}

通過以上代碼，你可以動態編譯和執行C#字符串中的代碼。請注意，這種方法可能存在一定的安全風險，需要謹慎使用。