DiagnosticDescriptor Class

var descriptor = new DiagnosticDescriptor("XML0001", "Missing end tag", "Missing end tag for '{0}'.", DiagnosticSeverity.Error);

// Create a diagnostic from the descriptor: the arguments fill the message template
var diagnostic = Diagnostic.Create(descriptor, Location.None, "root");
Assert.Equal("XML0001", diagnostic.Id);
Assert.Equal("Missing end tag for 'root'.", diagnostic.Message);
Assert.Equal(DiagnosticSeverity.Error, diagnostic.Severity);