C# 使用动态 lambda 但在选择要使用的方法时得到不明确的匹配

Question

根据 Pashov 的博客文章，我有以下静态方法用于在过滤器选项之间进行选择。

public static class ExpressionRetriever  
    {  
        private static MethodInfo containsMethod = typeof(string).GetMethod("Contains");  
        private static MethodInfo startsWithMethod = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });  
        private static MethodInfo endsWithMethod = typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) });



    public static Expression GetExpression<T>(ParameterExpression param, ExpressionFilter filter)  
    {  
        MemberExpression member = Expression.Property(param, filter.PropertyName);  
        ConstantExpression constant = Expression.Constant(filter.Value);  
        switch (filter.Comparison)  
        {  
            case Comparison.Equal:  
                return Expression.Equal(member, constant);  
            case Comparison.GreaterThan:  
                return Expression.GreaterThan(member, constant);  
            case Comparison.GreaterThanOrEqual:  
                return Expression.GreaterThanOrEqual(member, constant);  
            case Comparison.LessThan:  
                return Expression.LessThan(member, constant);  
            case Comparison.LessThanOrEqual:  
                return Expression.LessThanOrEqual(member, constant);  
            case Comparison.NotEqual:  
                return Expression.NotEqual(member, constant);  
            case Comparison.Contains:  
                return Expression.Call(member, containsMethod, constant);  
            case Comparison.StartsWith:  
                return Expression.Call(member, startsWithMethod, constant);  
            case Comparison.EndsWith:  
                return Expression.Call(member, endsWithMethod, constant);  
            default:  
                return null;  
        }  
    }  
}

一旦到达该行，return Expression.Call(member, startsWithMethod, constant);它就会抛出以下错误：

> System.TypeInitializationException
  HResult=0x80131534
  Message=The type initializer for 'ExpressionRetriever' threw an exception.
  Source=JobsLedger.API
  StackTrace:
   at JobsLedger.API.ControllerServices.Shared.OrderAndFIlterHelpers.DynamicFilteringHelper.ExpressionRetriever.GetExpression[T](ParameterExpression param, ExpressionFilter filter) in C:\AURELIA\1.0 - JobsLedgerSPA -ASPNET CORE 3.0\JobsLedger.API\ControllerServices\Shared\OrderAndFIlterHelpers\DynamicFilteringHelper.cs:line 84
   at JobsLedger.API.ControllerServices.Shared.OrderAndFIlterHelpers.DynamicFilteringHelper.ConstructAndExpressionTree[T](List`1 filters) in C:\AURELIA\1.0 - JobsLedgerSPA -ASPNET CORE 3.0\JobsLedger.API\ControllerServices\Shared\OrderAndFIlterHelpers\DynamicFilteringHelper.cs:line 39
   at JobsLedger.API.ControllerServices.Shared.ODataFilterAndSort.ParginatedFilteredSorted[T](IQueryable`1 source, String query) in C:\AURELIA\1.0 - JobsLedgerSPA -ASPNET CORE 3.0\JobsLedger.API\ControllerServices\Shared\OrderAndFIlterHelpers\ODataFilterAndSort.cs:line 41
   at JobsLedger.API.ControllerServices.API.App.ClientServices.ClientServices.GetPaginatedClients(String query) in C:\AURELIA\1.0 - JobsLedgerSPA -ASPNET CORE 3.0\JobsLedger.API\ControllerServices\API\App\ClientServices\ClientServices.cs:line 65
   at JobsLedger.API.Controllers.API.App.ClientController.Index(String query) in C:\AURELIA\1.0 - JobsLedgerSPA -ASPNET CORE 3.0\JobsLedger.API\Controllers\API\App\ClientController.cs:line 28
   at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()
> 
> Inner Exception 1: AmbiguousMatchException: Ambiguous match found.

如果我注释掉它private static MethodInfo containsMethod，private static MethodInfo endsWithMethod它现在可以工作了。

我发现这篇文章的答案（同类问题）说要使用这个重载，但它已经这样做了。

我怎样才能避免这种情况，以便它可以在不同的方法之间进行选择而不会看到模棱两可的选择？

Answer 1

虽然您的问题没有这么具体说明，但从提供的堆栈跟踪推断出您正在 .NET Core 环境中运行代码。

请注意，无法在 .NET Framework 环境中重现此错误。

在 .Net Core 中测试时，此行发生错误

private static MethodInfo containsMethod = typeof(string).GetMethod("Contains");  

请注意缺少用于缩小方法匹配范围的参数类型。

通过提供类似的参数类型

private static MethodInfo containsMethod = 
    typeof (string).GetMethod("Contains", new Type[]{typeof (string)});

错误已解决，代码按预期运行。

.Net 工作代码的小提琴