咨询区
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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
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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
