Unity PlayFab SDK异步转async/await时PlayFabResult类型转换错误

使用Unity PlayFab SDK时，希望将其异步回调风格的API转换为async/await用法以提升代码可读性，编写了泛型方法包装任意API调用为Task，但在错误回调中无法正确设置Error属性，遇到编译错误。

初始实现代码 #

public static class PFAsyncWorks
{
    public delegate void PlayFabMethod<TIn, TOut>( TIn r, Action<TOut> o , Action<PlayFabError> e, object c, Dictionary<string, string> h);
 
    public static Task<TOut> Run<TIn, TOut>( PlayFabMethod<TIn,TOut> playfabMethod,
        TIn request, object customData = null, Dictionary<string, string> extraHeaders = null )
    {
        var tcs = new TaskCompletionSource<TOut>();
 
        Action<TOut> resultCallback = result => tcs.SetResult( result );
        void errorCallback(PlayFabError error)
        {
            var ret = default(TOut);
            // 此行报错，注释后可运行但错误会被吞掉
            ((PlayFabResult<TOut>)ret).Error = error;
            tcs.SetResult(ret);
        }

        playfabMethod.Invoke( request, resultCallback, errorCallback, customData, extraHeaders );
 
        return tcs.Task;
    }
}

调用代码 #

var confirm = await PFAsyncWorks.Run<ConfirmPurchaseRequest, ConfirmPurchaseResult>(
             PlayFabClientAPI.ConfirmPurchase, new ConfirmPurchaseRequest() {OrderId = "sd" });  

遇到的错误 #

The type 'TOut' cannot be used as type parameter 'TResult' in the generic type or method 'PlayFabResult'. There is no implicit reference conversion from 'TOut' to 'PlayFab.SharedModels.PlayFabResultCommon'.

尝试添加泛型约束后的代码 #

public static Task<TOut> Run<TIn, TOut>( PlayFabMethod<TIn,TOut> playfabMethod,
    TIn request, object customData = null, Dictionary<string, string> extraHeaders = null )
    where TIn : PlayFabRequestCommon where TOut :  PlayFabResult<TOut>, new()
{
    var tcs = new TaskCompletionSource<TOut>();
    
    try
    {
        void resultCallback(TOut result)
        {
            tcs.TrySetResult(result);
        }

        void errorCallback(PlayFabError error)
        {
            Debug.LogError(error.GenerateErrorReport());
            
            TOut t = new TOut();
            
            t.Error = error;
            
            tcs.TrySetResult(t);
        }

        playfabMethod.Invoke(request, resultCallback, errorCallback, customData, extraHeaders);
    }
    catch(Exception ex)
    {
        tcs.TrySetException(ex);
    }
    
    return tcs.Task;
}

此版本仍报错：

The type 'TOut' cannot be used as type parameter 'TResult' in the generic type or method 'PlayFabResult'. There is no implicit reference conversion from 'TOut' to 'PlayFab.SharedModels.PlayFabResultCommon'.

解决方案 #

1. 修正泛型约束 #

问题出在错误的泛型约束上：PlayFab的所有具体结果类（如ConfirmPurchaseResult）都继承自PlayFabResultCommon，而非PlayFabResult<TOut>。PlayFabResult<T>是通用包装类，PlayFabResultCommon才是所有结果类型的基类，且包含Error属性。

修改后的泛型约束如下：

public static Task<TOut> Run<TIn, TOut>(PlayFabMethod<TIn, TOut> playfabMethod,
    TIn request, object customData = null, Dictionary<string, string> extraHeaders = null)
    where TIn : PlayFabRequestCommon
    where TOut : PlayFabResultCommon, new() // 正确的约束

2. 正确设置Error属性 #

修正约束后，即可直接在错误回调中创建TOut实例并设置Error属性：

void errorCallback(PlayFabError error)
{
    Debug.LogError(error.GenerateErrorReport());
    var errorResult = new TOut();
    errorResult.Error = error; // 现在可正常访问Error属性
    tcs.TrySetResult(errorResult);
}

3. 更符合async/await规范的错误处理（推荐） #

如果希望遵循C#异步编程最佳实践，建议在错误回调中直接设置Task的异常，而非返回带Error的结果对象。调用方可以通过try/catch捕获错误，逻辑更清晰：

修改错误回调：

void errorCallback(PlayFabError error)
{
    // 将PlayFabError包装为Exception，保留原始错误信息
    tcs.TrySetException(new Exception(error.GenerateErrorReport(), error));
}

调用时使用try/catch处理错误：

try
{
    var confirm = await PFAsyncWorks.Run<ConfirmPurchaseRequest, ConfirmPurchaseResult>(
        PlayFabClientAPI.ConfirmPurchase, new ConfirmPurchaseRequest() { OrderId = "sd" });
    // 处理成功结果
}
catch (Exception ex)
{
    // 取出原始PlayFabError
    var playFabError = ex.InnerException as PlayFabError;
    if (playFabError != null)
    {
        Debug.LogError(playFabError.GenerateErrorReport());
        // 针对不同错误类型做处理
    }
}

内容的提问来源于stack exchange，提问作者user3010678