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• Jader Dias

参考下面的代码：

myEnumerable.Select(a => ThisMethodMayThrowExceptions(a));

如何保证在 Linq 的查询过程中即使抛出了异常，查询不会被提前中断，就好像在每层迭代上都有默认的 try catch 块。

回答区

• LeBaptiste

我写了一个小的扩展方法，它可以实现在 IEnumerable<T> 中的每一层迭代上加上 try catch 异常处理逻辑, 扩展方法代码如下：

public static class OnCaughtExceptionExtension
{public static IEnumerable<SelectTryResult<TSource, TResult>> SelectTry<TSource, TResult>(this IEnumerable<TSource> enumerable, Func<TSource, TResult> selector){foreach (TSource element in enumerable){SelectTryResult<TSource, TResult> returnedValue;try{returnedValue = new SelectTryResult<TSource, TResult>(element, selector(element), null);}catch (Exception ex){returnedValue = new SelectTryResult<TSource, TResult>(element, default(TResult), ex);}yield return returnedValue;}}public static IEnumerable<TResult> OnCaughtException<TSource, TResult>(this IEnumerable<SelectTryResult<TSource, TResult>> enumerable, Func<Exception, TResult> exceptionHandler){return enumerable.Select(x => x.CaughtException == null ? x.Result : exceptionHandler(x.CaughtException));}public static IEnumerable<TResult> OnCaughtException<TSource, TResult>(this IEnumerable<SelectTryResult<TSource, TResult>> enumerable, Func<TSource, Exception, TResult> exceptionHandler){return enumerable.Select(x => x.CaughtException == null ? x.Result : exceptionHandler(x.Source, x.CaughtException));}public class SelectTryResult<TSource,TResult>{internal SelectTryResult(TSource source, TResult result, Exception exception){Source = source;Result = result;CaughtException = exception;}public TSource Source { get; private set; }public TResult Result { get; private set; }public Exception CaughtException { get; private set; }}
}

接下来就可以像下面这样使用了。

public void Test()
{List<string> completedProcesses = initialEnumerable.SelectTry(x => RiskyOperation(x)).OnCaughtException(exception => { _logger.Error(exception); return null; }).Where(x => x != null) // filter the ones which failed.ToList();
}

当然你有兴趣的话，还可以实现一个 SkipOnException 扩展方法，当在迭代中出现异常时自由选择是否可以跳过异常处理。

• THTP

如果你 select 的是 IQueryable 类型，这时候你可能需要在 Expression 上扩展而不是 Lambda，参考如下扩展方法。

public static class ExpressionHelper
{public static Expression<Func<TSource, TResult>> TryDefaultExpression<TSource, TResult>(Expression<Func<TSource, TResult>> success, TResult defaultValue){var body = Expression.TryCatch(success.Body, Expression.Catch(Expression.Parameter(typeof(Exception)), Expression.Constant(defaultValue, typeof (TResult))));var lambda = Expression.Lambda<Func<TSource, TResult>>(body, success.Parameters);return lambda;}
}

然后像下面这样使用。

[Test]
public void Test()
{var strings = new object [] {"1", "2", "woot", "3", Guid.NewGuid()}.AsQueryable();var ints = strings.Select(ExpressionHelper.TryDefaultExpression<object, int>(x => Convert.ToInt32(x), 0));Assert.IsTrue(ints.SequenceEqual(new[] {1, 2, 0, 3, 0}));
}

点评区

其实面对这种场景，我第一个想到的还是 Polly 框架，大家有兴趣可以看一看：https://github.com/App-vNext/Polly