What is a PatternSyntax in Roslyn

Question

This seems a fairly stupid question and I'm not sure it is suitable for StackOverflow.

A user of my tool has reported an error with the message

Unable to cast object of type
'Microsoft.CodeAnalysis.CSharp.Syntax.LiteralExpressionSyntax' to type
'Microsoft.CodeAnalysis.CSharp.Syntax.PatternSyntax'.

I don't know exactly what his code looks like (at least not yet). I might be able to reproduce the error if I could write some code containing a PatternSyntax.

My problem is, that I haven't a clue what language construct in C# generates a syntax node of type PatternSyntax.

Can anybody help me with an example?

Answer 1

I believe it's part of the syntax generated from Pattern Matching. The spec for this feature describes the grammar along with some more examples.

PatternSyntax is defined here: http://sourceroslyn.io/Microsoft.CodeAnalysis.CSharp/Generated/Syntax.xml.Internal.Generated.cs.html#b554fa43e6090c28

The following inherit from PatternSyntax

DiscardPatternSyntax
DeclarationPatternSyntax
VarPatternSyntax
RecursivePatternSyntax
ConstantPatternSyntax

I believe one example of VarPatternSyntax is var o in the following:

static object CreateShape(string shapeDescription)
{
    switch (shapeDescription)
    {
        case var o when (o?.Trim().Length ?? 0) == 0:
            // white space
            return null;
        default:
            return "invalid shape description";
    }            
}

We can take the above code and feed it into RoslynQuoter to see how to generate it and it would look something like:

CasePatternSwitchLabel(
            VarPattern(
                SingleVariableDesignation(
                    Identifier("o"))),
            Token(SyntaxKind.ColonToken))
        .WithWhenClause(
            WhenClause(
                BinaryExpression(
                    SyntaxKind.EqualsExpression,
                    ParenthesizedExpression(
                        BinaryExpression(
                            SyntaxKind.CoalesceExpression,
                            ConditionalAccessExpression(
                                IdentifierName("o"),
                                MemberAccessExpression(
                                    SyntaxKind.SimpleMemberAccessExpression,
                                    InvocationExpression(
                                        MemberBindingExpression(
                                            IdentifierName("Trim"))),
                                    IdentifierName("Length"))),
                            LiteralExpression(
                                SyntaxKind.NumericLiteralExpression,
                                Literal(0)))),
                    LiteralExpression(
                        SyntaxKind.NumericLiteralExpression,
                        Literal(0)))))))