Resolve errors and warnings that involve async, await and the Task-asynchronous protocol - C# reference (original) (raw)

This article covers the following compiler errors:

• CS1983: Since this is an async method, the return expression must be of type 'Task<T>' rather than 'T'.
• CS1985: Cannot await in a catch clause.
• CS1986: 'await' requires that the type have a suitable 'GetAwaiter' method.
• CS1989: Async lambda expressions cannot be converted to expression trees.
• CS1991: 'Type' cannot implement 'event' because it is a Windows Runtime event and 'event' is a regular .NET event.
• CS1992: The 'await' operator can only be used when contained within a method or lambda expression marked with the 'async' modifier.
• CS1994: The 'async' modifier can only be used in methods that have a body.
• CS1995: The 'await' operator may only be used in a query expression within the first collection expression of the initial 'from' clause or within the collection expression of a 'join' clause.
• CS1996: Cannot await in the body of a lock statement.
• CS1997: Since function is an async method that returns a value, a return keyword must not be followed by an object expression.
• CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
• CS4008: Cannot await 'void'.
• CS4009: A void or int returning entry point cannot be async.
• CS4014: Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the await operator to the result of the call.
• CS4032: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task<T>'.
• CS4033: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'.
• CS8892: Method will not be used as an entry point because a synchronous entry point was found.
• CS9123: The '&' operator should not be used on parameters or local variables in async methods.
• CS9330: 'MethodImplAttribute.Async' cannot be manually applied to methods. Mark the method 'async'.

Await expression requirements

• CS1985: Cannot await in a catch clause.
• CS1986: 'await' requires that the type have a suitable 'GetAwaiter' method.
• CS1992: The 'await' operator can only be used when contained within a method or lambda expression marked with the 'async' modifier.
• CS1995: The 'await' operator may only be used in a query expression within the first collection expression of the initial 'from' clause or within the collection expression of a 'join' clause.
• CS1996: Cannot await in the body of a lock statement.
• CS4008: Cannot await 'void'.
• CS4032: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task<T>'.
• CS4033: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'.

To use the await operator correctly, follow these rules. For more information, see Asynchronous programming with async and await.

• Don't use await in catch clauses (CS1985). While you can use await in try blocks and finally blocks (in C# 6 and later), catch blocks present special challenges with exception handling and control flow.
• Don't use await inside lock statement blocks (CS1996). The compiler doesn't support this to avoid emitting code prone to deadlocks.
• Use await only in specific locations within query expressions (CS1995): within the first collection expression of the initial from clause, or within the collection expression of a join clause.
• Mark methods or lambda expressions with the async modifier before using await (CS1992, CS4032, CS4033).
• Ensure awaited types have an accessible GetAwaiter method that returns an awaiter type (CS1986).
• Don't apply await to expressions of type void (CS4008).
• Change the return type to Task for methods that don't return a value, or Task<T> for methods that return a value.

Async method signature requirements

• CS1983: Since this is an async method, the return expression must be of type 'Task<T>' rather than 'T'.
• CS1994: The 'async' modifier can only be used in methods that have a body.
• CS4009: A void or int returning entry point cannot be async.
• CS8892: Method will not be used as an entry point because a synchronous entry point was found.
• CS9330: 'MethodImplAttribute.Async' cannot be manually applied to methods. Mark the method 'async'.

To declare async methods correctly, follow these signature requirements. For more information, see Async main return values.

• Return one of the valid types: void, Task, Task<T>, a task-like type, IAsyncEnumerable, or IAsyncEnumerator (CS1983).
• Use the async modifier only on methods with a body (CS1994). Remove the async modifier on abstract methods in interfaces or classes.
• Update to C# 7.1 or higher to use async on the Main entry point, or avoid using async on entry points in earlier versions (CS4009).
• Remove synchronous entry points if you have both sync and async entry points (CS8892).
• Use the async keyword instead of manually applying MethodImplAttribute.Async (CS9330).

Async practices

• CS1989: Async lambda expressions cannot be converted to expression trees.
• CS1991: 'Type' cannot implement 'event' because it is a Windows Runtime event and 'event' is a regular .NET event.
• CS1997: Since function is an async method that returns a value, a return keyword must not be followed by an object expression.
• CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
• CS4014: Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the await operator to the result of the call.
• CS9123: The '&' operator should not be used on parameters or local variables in async methods.

To write async code correctly and avoid common pitfalls, follow these best practices. For more information, see Asynchronous programming with async and await.

• Always await calls to async methods that return Task or Task (CS4014). Unawaited calls can lead to lost exceptions and unexpected behavior.
• Don't return a value from async methods that return Task (non-generic); use Task<T> instead (CS1997).
• Include at least one await operator in async methods, or remove the async modifier (CS1998).
• Remove the return statement if the method should return Task (CS1997, CS1998).
• Change the method's return type to Task<T> to return a value (CS1997, CS1998).
• Remove the async modifier and return the task directly if you don't need the async state machine (CS1997, CS1998).
• Don't use async methods in expression trees (CS1989). Expression trees represent code as data and don't support the complex state machine transformations required by async methods.
• Don't mark add or remove accessors in an interface or WinRT event as async (CS1991). This is a platform-specific restriction for Windows Runtime interoperability.
• Avoid using the address-of operator (&) on expressions inside async methods (CS9123). The target may be relocated in memory during suspension, making the pointer invalid.