c# – Linq.Expression TryCatch – 将异常传递给Catch Block？

2019年7月28日 248次阅读

所以我一直在修补
Linq.Expressions(如果有人可以建议一个更正确或更优雅的方式来做我正在做的事情,请随意插入)并且试图做某事时碰壁.

让我们假设我们有一个简单的数学课：

public class SimpleMath {
    public int AddNumbers(int number1, int number2) {           
        return number1 + number2;
    }
}

我决定将AddNumbers方法转换为简单的Func< object,object,object>代表.

为此,我执行以下操作：

// Two collections, one for Type Object paramaters and one for converting to Type int.
List<ParameterExpression> parameters = new List<ParameterExpression>();
List<Expression> convertedParameters = new List<Expression>();

// Populate collections with Parameter and conversion
ParameterExpression parameter1 = Expression.Parameter(typeof(object));
parameters.Add(parameter1);
convertedParameters.Add(Expression.Convert(parameter1, typeof(int)));

ParameterExpression parameter2 = Expression.Parameter(typeof(object));
parameters.Add(parameter2);
convertedParameters.Add(Expression.Convert(parameter2, typeof(int)));

// Create instance of SimpleMath
SimpleMath simpleMath = new SimpleMath();

// Get the MethodInfo for the AddNumbers method
MethodInfo addNumebrsMethodInfo = simpleMath.GetType().GetMethods().Where(x => x.Name == "AddNumbers").ToArray()[0];
// Create MethodCallExpression using the SimpleMath object, the MethodInfo of the method we want and the converted parameters
MethodCallExpression returnMethodWithParameters = Expression.Call(Expression.Constant(simpleMath), addNumebrsMethodInfo, convertedParameters);

// Convert the MethodCallExpression to return an Object rather than int
UnaryExpression returnMethodWithParametersAsObject = Expression.Convert(returnMethodWithParameters, typeof(object));

// Create the Func<object, object, object> with our converted Expression and Parameters of Type Object
Func<object, object, object> func = Expression.Lambda<Func<object, object, object>>(returnMethodWithParametersAsObject, parameters).Compile();
    object result = func(20, 40); // result = 60

因此,如果您运行该代码,则func应返回简单计算.但是,它接受Type对象的参数,这显然会使它在运行时遇到问题,例如：

object result1 = func(20, "f"); // Throws InvalidCastException

所以我想把这个方法包装在Try … Catch中(显然,如果我们处理对AddNumbers的直接调用并将字符串作为参数传递,那么在编译时会得到这个确切的问题).

因此,为了捕获此异常,我可以执行以下操作：

TryExpression tryCatchMethod = TryExpression.TryCatch(returnMethodWithParametersAsObject, Expression.Catch(typeof(InvalidCastException), Expression.Constant(55, typeof(object))));
Func<object, object, object> func = Expression.Lambda<Func<object, object, object>>(tryCatchMethod, parameters).Compile();
object result = func(20, "f"); // result = 55

TryExpression.TryCatch接受一个Expression主体,然后是一个CatchBlock处理程序的集合. returnMethodWithParametersAsObject是我们希望包装的表达式,Expression.Catch定义我们要捕获的Exception是Type InvalidCastException,它的Expression主体是常量,55.

因此处理异常,但除非我想在抛出异常时始终返回静态值,否则它没有多大用处.所以返回SimpleMath类我添加了一个新方法HandleException：

public class SimpleMath {
    public int AddNumbers(int number1, int number2) {
        return number1 + number2;
    }

    public int HandleException() {
        return 100;
    }
}

按照上面的相同过程,我将新方法转换为Expression：

MethodInfo handleExceptionMethodInfo = simpleMath.GetType().GetMethods().Where(x => x.Name == "HandleException").ToArray()[0];
MethodCallExpression returnMethodWithParameters2 = Expression.Call(Expression.Constant(simpleMath), handleExceptionMethodInfo);
UnaryExpression returnMethodWithParametersAsObject2 = Expression.Convert(returnMethodWithParameters2, typeof(object));

然后在创建TryCatch块时使用它：

TryExpression tryCatchMethod2 = TryExpression.TryCatch(returnMethodWithParametersAsObject, Expression.Catch(typeof(InvalidCastException), returnMethodWithParametersAsObject2));
Func<object, object, object> func = Expression.Lambda<Func<object, object, object>>(tryCatchMethod2, parameters).Compile();
object result = func(20, "f"); // result = 100

所以这次抛出InvalidCastException时,将执行SimpleMath.HandleException方法.到目前为止一切顺利,我现在可以在出现异常时执行一些代码.

我现在的问题是,在一个正常的内联Try … Catch块中你实际上有你可以使用的异常对象.例如.

try {
    // Do stuff that causes an exception
} catch (InvalidCastException ex) {
    // Do stuff with InvalidCastException ex
}

我可以在抛出异常时执行代码,但我似乎无法弄清楚如何实际上如同在正常的Try … Catch块中那样触及异常对象.

任何帮助,将不胜感激！

附：我知道你实际上并没有按照我上面的方式组织任何事情,但我认为有必要举例说明我想做什么的机制.

最佳答案 您需要将捕获的异常作为参数传递给CatchBlock表达式.

为此你应该这样做：

>更改HandleException的签名.它将以异常作为参数：

public int HandleException(InvalidCastException exp)
{
    // Put here some real logic. I tested it using line below
    Console.WriteLine(exp.Message);
    return 100;
}

>使用CatchBlock.Variable将处理的异常传递给catch块.您可以设置它使用构造函数.阅读以下代码中的评论：

    // Create parameter that will be passed to catch block 
    var excepParam = Expression.Parameter(typeof(InvalidCastException));

    MethodInfo handleExceptionMethodInfo = simpleMath.GetType().GetMethods().Where(x => x.Name == "HandleException").ToArray()[0];
    MethodCallExpression returnMethodWithParameters2 = Expression.Call(Expression.Constant(simpleMath), handleExceptionMethodInfo, excepParam);
    UnaryExpression returnMethodWithParametersAsObject2 = Expression.Convert(returnMethodWithParameters2, typeof(object));

    // Put created parameter before to CatchBlock.Variable using Expression.Catch 
    // that takes the first argument as ParameterExpression
    TryExpression tryCatchMethod2 = TryExpression.TryCatch(returnMethodWithParametersAsObject, Expression.Catch(excepParam, returnMethodWithParametersAsObject2));
    var exppp = Expression.Lambda<Func<object, object, object>>(tryCatchMethod2, parameters);
    Func<object, object, object> func2 = Expression.Lambda<Func<object, object, object>>(tryCatchMethod2, parameters).Compile();
    object result2 = func2(20, "f"); // result = 100
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