Background and Motivation

The parameter null checking feature #36024 needs some new public APIs and behavior changes to public API.

Proposed API

namespace Microsoft.CodeAnalysis
{
     public interface IParameterSymbol
     {
+        /// <summary>
+        /// True if the compiler will synthesize a null check for this parameter (the parameter is declared in source with a <c>!!</c> following the parameter name).
+        /// </summary>
+        bool IsNullChecked { get; }
     }
}

namespace Microsoft.CodeAnalysis.CSharp
{
    public enum SyntaxKind
    {
        // ...

        /// <summary>Represents <c>??=</c> token.</summary>
        QuestionQuestionEqualsToken = 8284,
+        /// <summary>Represents <c>!!</c> token.</summary>
+        ExclamationExclamationToken = 8285,
    }
}

namespace Microsoft.CodeAnalysis.CSharp.Syntax
{
    public sealed class ParameterSyntax
    {
+        public SyntaxToken ExclamationExclamationToken { get; }
    }
}

Usage Examples

The IDE uses these APIs to properly support the feature. For example, the refactoring to add a null check for a parameter:

Alternative Designs

One alternative would be to eliminate IParameterSymbol.IsNullChecked, and force public users to go through syntax and check the ParameterSyntax.ExclamationExclamationToken to determine if a parameter is null checked. It seems inconvenient, especially when it comes to IOperation scenarios, where the user may not be referencing MS.CA.CSharp but still want to know if a parameter was null checked.

Risks

Not aware of any risks from the current design.

Control flow graph

In the current feature implementation, the IOperation behavior is the same whether or not parameter null checking is used. Users are expected to grab the parameter symbol and check its IsNullChecked property if they care about it. This seems consistent with IOperation behaviors in other "implicit code" scenarios.

However, the control flow graph behavior does currently differ depending on whether parameter null checking is used. Nodes are inserted to represent where the null check occurs and what the semantics of the null check roughly are. Should the API do this? Or should the graph be the same regardless of whether parameter null-checking is used, leaving users to check the IParameterSymbol.IsNullChecked API and handle appropriately if they need to?