MustAwaitAnalyzer

Overview

MustAwaitAnalyzer is a roslyn analyzer that enforces the use of await when calling methods that return:

• System.Threading.Tasks.Task
• Cysharp.Threading.Tasks.UniTask

Install into Unity Project

Requires Unity 2021.1.2f1 or later. You can add https://github.com/DeNA/MustAwaitAnalyzer.git?path=com.dena.must-await-analyzer#1.0.0 to Package Manager.

Install into .NET Project

You can install from NuGet packages.

DENA008: Must use await

Item	Value
Category	DenaUnityAnalyzers
Enabled	True
Severity	Error
CodeFix	False

Analysis Targets

• Method calls, property references, and instance creations whose evaluation result is one of the following types:
  • System.Threading.Tasks.Task
  • Cysharp.Threading.Tasks.UniTask
  • System.Threading.Tasks.Task<T>
  • Cysharp.Threading.Tasks.UniTask<T>

Examples are provided below:

// bad
namespace BadCase
{
    public class Task1
    {
        Task Task_NoAsync() => Task.Run(() => {}); // DENA008, "Must use await" is reported

        public Task UseNoAsync()
        {
            Task_NoAsync(); // DENA008, "Must use await" is reported
        }
        
        Action<object> action = (object obj) => {};
    
        public async Task Hoge()
        {
            new Task(action, "a"); // DENA008, "Must use await" is reported
        }
    }
}

// good
namespace GoodCase
{
    public class Task1
    {
        async Task Task_NoAsync() => await Task.Run(() => {}); // DENA008, "Must use await" is not reported

        public async Task UseNoAsync()
        {
            await Task_NoAsync(); // DENA008, "Must use await" is not reported
        }
        
        Action<object> action = (object obj) => {};
    
        public async Task Hoge()
        {
            await new Task(action, "a"); // DENA008, "Must use await" is not reported
        }
    }
}

NOTE1: The following types are not analyzed:

• UniTaskVoid
• ValueTask

NOTE2: If the last method in a method chain is named Terminate or Forget, the analysis does not take place.

static async UniTask HogeAsync()
{
    await Task.Run(() => {});
}

static void TerminateCase()
{
    HogeAsync(); // DENA008 is reported due to missing await
    HogeAsync().GetAwaiter(); // DENA008 is reported due to missing await
    HogeAsync().GetAwaiter().Terminate(); // Nothing is reported
    HogeAsync().GetAwaiter().Forget(); // Nothing is reported
}

NOTE3: If a method call directly references a property that is an analysis target, the property reference is analyzed instead of the method call.

// bad
public async static Task<Task> BadPropertyCase()
{
    var p = new Program();
    p.Example(); // DENA008 is reported
    return p.Example().Result; // Result returns Task, hence DENA008 is reported
}

public async Task<Task> Example()
{
    return await new Task<Task>(null);
}

// good
public async static Task<int> GoodPropertyCase()
{
    var p = new Program();
    p.Example(); // DENA008 is reported
    return p.Example().Result; // Result returns int, hence DENA008 is not reported
}

public async Task<int> Example()
{
    return await new Task<int>(null);
}

NOTE4: The following method calls, property references, and field references are excluded from analysis:

• System.Threading.Tasks.Task.CompletedTask
• System.Threading.Tasks.Task.FromResult
• Cysharp.Threading.Tasks.UniTask.CompletedTask
• Cysharp.Threading.Tasks.UniTask.FromResult

NOTE5: If Task or UniTask objects are assigned to a variable and then awaited, no warning is issued. Additionally, if Task or UniTask objects are stored in a list (a class that implements System.Collections.Generic.IList) and awaited collectively, no warning is issued either.

// good
public async Task M1()
{
    var task = Task.Run(() => {}); // DENA008 is not reported
    await task;
}

// good
public async Task M1()
{
    var list = new List<Task>();
    list.Add(Task.Run(() => {})); // DENA008 is not reported
    list.Add(Task.Run(() => {})); // DENA008 is not reported
    await Task.WhenAll(list);
}

Analysis Locations

If the target of analysis exists within the following syntax and lacks an await, DENA008 will be reported.

Within a Block

// bad
namespace BadCase
{
    public class Task1
    {
        Task Task_NoAsync() => Task.Run(() => {}); // DENA008, "Must use await" is reported

        public Task UseNoAsync()
        {
            Task_NoAsync(); // DENA008, "Must use await" is reported
        }
    }
}

// good
namespace GoodCase
{
    public class Task1
    {
        async Task Task_NoAsync() => await Task.Run(() => {});

        public async Task UseNoAsync()
        {
            await Task_NoAsync();
        }
    }
}

Within a Statement

Examples for various statement types are shown below.

• using Statement

// bad

namespace BadCase
{
    public class Task1
    {
        public async void M1() 
        {
            using (M2()) // DENA008, "Must use await" is reported
            {
            }
        }
        
        public async Task<IDisposable> M2()
        {
            return await (Task<IDisposable>)Task.Run(() => {});
        }
    }
}

// good
namespace GoodCase
{
    public class Task1
    {
        public async void M1() 
        {
            using (await M2()) // DENA008 is not reported
            {
            }
        }
        
        public async Task<IDisposable> M2()
        {
            return await (Task<IDisposable>)Task.Run(() => {});
        }
    }
}

• do-while Statement

// bad
public class BadCase {
    public async void M() 
    {
        do 
        {
            Task.Run(() => {}); // DENA008, "Must use await" is reported.
        }
        while (true);
        {
        }
    }
}

// good
public class GoodCase {
    public async void M() 
    {
        do 
        {
            await Task.Run(() => {}); // DENA008 is not reported.
        }
        while (true);
        {
        }
    }
}

• if Statement

// bad case
namespace FailedCase
{
    public class C {
        public async void M() 
        {
            if (M2()) // DENA008 is reported
            {
            }
        }

        public async Task<bool> M2()
        {
            return await (Task<bool>)Task.Run(() => {});
        }
    }
}

// good case
namespace SuccessCase
{
    public class C {
        public async void M() 
        {
            if (await M2())
            {
            }
        }

        public async Task<bool> M2()
        {
            return await (Task<bool>)Task.Run(() => {});
        }
    }
}

• yield Statement

// bad
namespace BadCase
{
    public class YieldReturn
    {
        public async IAsyncEnumerable<int> M1()
        {
            for (int i = 0; i < 10; i++)
            {
                yield return (Task<int>)Task.Run(() => 0); // DENA008 and CS0029 are reported
            }
        }
    }
}

// good
namespace GoodCase
{
    public class YieldReturn
    {
        public async IAsyncEnumerable<int> M1()
        {
            for (int i = 0; i < 10; i++)
            {
                yield return await (Task<int>)Task.Run(() => 0);
            }
        }
    }
}

Lambda

// bad
namespace FailedCase
{
    public class Task1
    {
        Func<Task<int>> Example() => (() => Task.Run(() => 0)); // DENA008, "Must use await" is reported

        public void UseAsync()
        {
            Example(); // DENA008 is not reported since the return type is Func<Task<int>>, not Task<int>.
        }
    }
}

// good
namespace SuccessCase
{
    public class Task1
    {
        Func<Task<int>> Example() => (async () => await Task.Run(() => 0));

        public void UseAsync()
        {
            Example(); // DENA008 is not reported since the return type is Func<Task<int>>, not Task<int>.
        }
    }
}

Asynchronous Processing within Partial Methods

namespace BadCase
{
    interface IStudent
    {
        Task GetName();
    }

    partial class PartialMethod : IStudent
    {
        public virtual partial Task GetName();
    }

    public partial class PartialMethod
    {
        public virtual partial async Task GetName()
        {
            Task.Run(() => {}); // DENA008 is reported.
        }
    }
}

namespace GoodCase
{
    interface IStudent
    {
        Task GetName();
    }

    partial class PartialMethod : IStudent
    {
        public virtual partial Task GetName();
    }

    public partial class PartialMethod
    {
        public virtual partial async Task GetName()
        {
            await Task.Run(() => {}); // DENA008, "Must use await" is not reported
        }
    }
}

Target-typed Inference

// badcase
namespace BadCase
{
    public class TargetTypedNewExpression
    {
        Func<object, int> func = null;

        public async Task<int> Hoge()
        {
            return await new Task<int>(func, "a");
        }

        public async void M()
        {
            Dictionary<string, int> field = new()
            {
                { "item1", Hoge() } // DENA008 is reported
            };
        }
    }
}

// GoodCase
namespace GoodCase
{
    public class TargetTypedNewExpression
    {
        Func<object, int> func = null;

        public async Task<int> Hoge()
        {
            return await new Task<int>(func, "a");
        }

        public async void M()
        {
            Dictionary<string, int> field = new()
            {
                { "item1", await Hoge() } // DENA008, "Must use await" is not reported
            };
        }
    }
}