Errors and warnings associated with source generators and interceptors

The following errors are generated when source generators or interceptors are loaded during a compilation:

• CS9137: The 'interceptors' experimental feature is not enabled. Add <Features>InterceptorsPreview<Features> to your project.
• CS9138: Method cannot be used as an interceptor because it or its containing type has type parameters.
• CS9139: Cannot intercept: compilation does not contain a file with path.
• CS9140: Cannot intercept: compilation does not contain a file with path. Did you mean to use a different path?
• CS9141: The provided line and character number does not refer to an interceptable method name, but rather to a token.
• CS9142: The given file has n lines, which is fewer than the provided line number m.
• CS9143: The given line is c characters long, which is fewer than the provided character number n.
• CS9144: Cannot intercept method M with interceptor V because the signatures do not match.
• CS9145: Cannot intercept: Path is unmapped. Expected mapped path.
• CS9146: An interceptor method must be an ordinary member method.
• CS9147: The provided line and character number does not refer to the start of a token. Did you mean to use line n and character c?
• CS9148: Interceptor must have a this parameter matching parameter.
• CS9149: Interceptor must not have a this parameter because method does not have a this parameter.
• CS9150: Interceptor cannot have a null file path.
• CS9151: Possible method name M cannot be intercepted because it is not being invoked.
• CS9152: Cannot intercept a call in file with this path because multiple files in the compilation have this path.
• CS9153: The indicated call is intercepted multiple times.
• CS9155: Cannot intercept call with M because it is not accessible within V.
• CS9156: Cannot intercept call to M with V because of a difference in 'scoped' modifiers or [UnscopedRef] attributes.
• CS9157: Line and character numbers provided to InterceptsLocationAttribute must be positive.
• CS9160: A nameof operator cannot be intercepted.
• CS9161: An interceptor cannot be marked with UnmanagedCallersOnlyAttribute.
• CS9177: Interceptor must be non-generic or have matching arity.
• CS9178: Method must be non-generic to match
• CS9206: An interceptor cannot be declared in the global namespace.
• CS9207: Cannot intercept because method is not an invocation of an ordinary member method.

The following warnings are generated when source generators or interceptors are loaded during a compilation:

• CS9154: Intercepting a call to M with interceptor V, but the signatures do not match.
• CS9158: Nullability of reference types in return type doesn't match interceptable method.
• CS9159: Nullability of reference types in type of parameter doesn't match interceptable method.

These errors and warnings follow these themes:

Interceptors are experimental

This error indicates you must enable the experimental feature.

• CS9137: The 'interceptors' experimental feature is not enabled. Add <Features>InterceptorsPreview<Features> to your project.

In C# 12, interceptors are experimental. Interceptors are subject to breaking changes or removal in a future release. Therefore, it is not recommended for production or released applications.

In order to use interceptors, you must set the <Features>InterceptorsPreview<Features> element in your project file. Without this flag, interceptors are disabled, even when other C# 12 features are enabled.

Signature mismatch

The following errors indicate a mismatch between the interceptor method and the interceptable method, or a violation of the rules regarding interceptor method declarations:

• CS9144: Cannot intercept method M with interceptor V because the signatures do not match.
• CS9148: Interceptor must have a this parameter matching parameter.
• CS9149: Interceptor must not have a this parameter because method does not have a this parameter.
• CS9155: Cannot intercept call with M because it is not accessible within V.
• CS9156: Cannot intercept call to M with V because of a difference in 'scoped' modifiers or [UnscopedRef] attributes.
• CS9177]: Interceptor must be non-generic or have matching arity.
• CS9178: Method must be non-generic to match

In addition, the following warnings indicate a mismatch in the signatures of the interceptor and the interceptable method:

• CS9154: Intercepting a call to M with interceptor V, but the signatures do not match.
• CS9158: Nullability of reference types in return type doesn't match interceptable method.
• CS9159: Nullability of reference types in type of parameter doesn't match interceptable method.

The interceptor method must be compatible with the interceptable method. You must follow these rules:

• Instance methods can intercept instance methods, not static methods. Similarly, static methods can only intercept static methods, not instance methods.
• The method signatures for the interceptor and the interceptable method must match: They must have the same parameters with the same modifiers in the same order. The return types must also match.
• The ref safe contexts must match. In other words, corresponding ref parameters must be either scoped or not scoped.
• They methods must both be non-generic, or both must have the same number of type parameters.

Incorrect mapping

Interceptors require a source mapping that maps the interceptable method and the interceptor method. The following errors indicate an issue with the mapping:

• CS9139: Cannot intercept: compilation does not contain a file with path.
• CS9140: Cannot intercept: compilation does not contain a file with path. Did you mean to use a different path?
• CS9141: The provided line and character number does not refer to an interceptable method name, but rather to a token.
• CS9142: The given file has n lines, which is fewer than the provided line number m.
• CS9143: The given line is c characters long, which is fewer than the provided character number n.
• CS9145: Cannot intercept: Path is unmapped. Expected mapped path.
• CS9147: The provided line and character number does not refer to the start of a token. Did you mean to use line n and character c?
• CS9150: Interceptor cannot have a null file path.
• CS9157: Line and character numbers provided to InterceptsLocationAttribute must be positive.

An interceptor specifies the location in the source code of the interceptable method. You specify the location by applying an [InterceptsLocation] attribute. You specify the line and column numbers in a remapped source file where the interceptor should be injected. These errors indicate that something in the attribute or the location doesn't match a location for a valid interceptable method. For details on the format and values for this attribute, see the feature specification.

Other failures

These errors indicate other limitations for interceptors:

• CS9138: Method cannot be used as an interceptor because it or its containing type has type parameters.
• CS9146: An interceptor method must be an ordinary member method.
• CS9151: Possible method name M cannot be intercepted because it is not being invoked.
• CS9152: Cannot intercept a call in file with this path because multiple files in the compilation have this path.
• CS9153: The indicated call is intercepted multiple times.
• CS9160: A nameof operator cannot be intercepted.
• CS9161: An interceptor cannot be marked with UnmanagedCallersOnlyAttribute.
• CS9206: An interceptor cannot be declared in the global namespace.
• CS9207: Cannot intercept because method is not an invocation of an ordinary member method.

These errors indicate that your interceptor method violates one of the rules for interceptors:

• Interceptors can't be generic methods, or members of generic classes.
• Interceptors must be ordinary members. They can't be operators, instance or static constructors, or finalizers.
• Interceptable methods must be ordinary members. They can't be operators, instance or static constructors, or finalizers, nor delegate invocations.
• Interceptable methods that are never invoked can't be intercepted.
• Interceptable methods can be intercepted at most once.
• Interceptors can't be methods limited to unmanaged callers.
• Interceptors must be contained in a namespace.