gistfile1.cs

var tree = CSharpSyntaxTree.ParseText(@"
	public class MyClass 
	{
		int MyMethod() { return 0; }
	}");

var Mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
var compilation = CSharpCompilation.Create("MyCompilation",
	syntaxTrees: new[] { tree }, references: new[] { Mscorlib });
//Note that we must specify the tree for which we want the model.
//Each tree has its own semantic model
var model = compilation.GetSemanticModel(tree);