In Roslyn API what the Speculative Semantic Model is? - roslyn

Question

What does the extension function SemanticModel.TryGetSpeculativeSemanticModel return? What is it good for?
I could not find any meaningful documentation on the subject.

Answer 1

The documentation for TryGetSpeculativeSemanticModel says:
Get a SemanticModel object that is associated with X that did not appear in this source code. This can be used to get detailed semantic information about sub-parts of X that did not appear in source code.
The analyzer code for the StyleCop SX1101 diagnostic offers a great example of this API's usage. SX1101 tells you that it's safe to remove a this. qualifier from your code.
Lets step through a slightly simplified version of the analyzer code:
var memberAccessExpression = (MemberAccessExpressionSyntax)ctx.Node.Parent;
var originalSymbolInfo = context.SemanticModel.GetSymbolInfo(
memberAccessExpression,
context.CancellationToken);
var statement = context.Node.FirstAncestorOrSelf<StatementSyntax>();
var annotation = new SyntaxAnnotation();
var speculationRoot = statement.ReplaceNode(
memberAccessExpression,
memberAccessExpression.Name.WithAdditionalAnnotations(annotation));
context.Node is a ThisExpressionSyntax. speculationRoot is an expression where we replaced this.SomeMember with SomeMember. The annotation is used for quick lookup later.
Now that we have generated a modified version of the code, we want to check if it would a) still compile, and b) still refer to the same thing. Since the sources in Roslyn are immutable, we'd need to compile the entire project again (which would be doable, but more expensive) to get a new SemanticModel for the changed code.
Here is where TryGetSpeculativeSemanticModel steps in:
if(!context.SemanticModel.TryGetSpeculativeSemanticModel(
statement.SpanStart,
speculationRoot,
out var speculativeModel)) return;
var mappedNode = speculationRoot.GetAnnotatedNodes(annotation).Single();
var newSymbolInfo = speculativeModel.GetSymbolInfo(mappedNode, context.CancellationToken);
So now if we manage to get a speculative semantic model, it means SomeMember was valid at that same position in code as this.SomeMember. We've used the annotation to quickly look up the SomeMember syntax node and then get it's semantic info from the speculativeModel.
All that's left to do now is to check whether the modified statement means the same thing as the original one.
if (!Equals(originalSymbolInfo.Symbol, newSymbolInfo.Symbol)) return;
context.ReportDiagnostic(Diagnostic.Create(Descriptor, context.Node.GetLocation()));