如何使用表达式调用以通用列表作为参数的方法调用？ [英] How to use Expressions to invoke a method call with a generic list as the parameter?
问题描述
我们在我们的项目中使用了非常出色的 ToStringBuilder 作为我们ToString的高性能，通用支持实现。它调试工作正常，直到我需要生成一个对象图形的字符串表示，以检查它是否在加载和关闭之间更改。以前我使用了一个MemoryStream将对象写到xml，但是这似乎重量级，所以我决定尝试使用ToStringBuilder，这是我打开一个showstopper ...
We're using the very Excellent ToStringBuilder in our project as a performant, generic backing for our ToString implementations. It worked fine for debugging until I needed to generate a string representation of an object graph to check if it had changed in between loading and closing. Previously I had used a MemoryStream to write the object out to xml, but this seemed heavyweight so I decided to try out using ToStringBuilder, which is where I hit a showstopper...
Our object graph uses generic typed lists heavily, so when the lists are printed out they look like the following:
PropertyName:{System.Collections.Generic.List`1[Namespace.Path.To.MyClassDto]}
<通过列表枚举和调用ToString对每个对象，这是罚款的默认行为（btw，ToStringBuilder支持object []，但我们不想改造我们的整个Dto层只是为了解决这个问题）。
Instead of enumerating through the list and invoking ToString on each object, which is fine as that's default behaviour (btw, ToStringBuilder supports object[], but we don't want to retrofit our entire Dto layer just to fix this problem).
我试图修补有问题的代码（ ToStringBuilder.cs ，第177行）来识别何时类型是一个通用列表，然后调用string.Join（，，list），但我不能得到我的头如何linq反射API处理泛型。
I tried to patch the code in question (ToStringBuilder.cs, line 177) to recognise when the type is a generic list, and then invoke string.Join(", ", list), but I couldn't get my head around how the linq reflection API handles generics.
我尝试的第一件事是获取String.Join（IEnumerable <>）方法的句柄，如下所示：
The first thing I tried was to get a handle to the String.Join(IEnumerable<>) method like this:
var stringJoinMethod = typeof(string).GetMethod("Join", new[] { typeof(string), typeof(IEnumerable<>) });
但是GetMethod返回null，所以没有工作。我最终发现这个StackOverflow问题，显示如何通过签名（调用getmethods（））获取一个通用方法，并过滤结果）。这给了我正确的方法句柄，所以我试着做这样的：
But GetMethod returned null so that didn't work. I eventually found this StackOverflow question that showed me how to get a generic method by signature (call getmethods() instead and filter the results). That got me the correct method handle, so I tried to do something like this:
private void AppendMember(MemberInfo memberInfo)
{
AppendQuotesIfRequiredForType(memberInfo);
Type type = GetMemberType(memberInfo);
var memberAppendMethod = typeof(StringBuilder).GetMethod("Append", new[] { type });
Expression getMemberValue = Expression.MakeMemberAccess(TargetArgExpression, memberInfo);
if (type.IsValueType)
{
Type appendArgType = memberAppendMethod.GetParameters()[0].ParameterType;
if (type != appendArgType)
{
getMemberValue = Expression.TypeAs(getMemberValue, typeof(object));
}
//my code begins here.
_appendExpressions.Add(Expression.Call(SbArgExpression, memberAppendMethod, getMemberValue));
}
else if (type.IsGenericType && (type.GetGenericTypeDefinition() == typeof(List<>)))
{
// now to emit some code to do the below, you wouldn't think it'd be this hard...
// string.Join(", ", genericList);
AppendStartOfMembers();
//this returns null, because generics are not well supported by the reflection API, boo!
var stringJoinMethod = typeof(string).GetGenericMethod("Join", new[] { typeof(string), typeof(IEnumerable<>) });
var CommaSpace = Expression.Constant(", ");
// this doesn't work, throws an ArgumentException as below
getMemberValue = Expression.Call(stringJoinMethod, CommaSpace, getMemberValue);
_appendExpressions.Add(Expression.Call(SbArgExpression, memberAppendMethod, getMemberValue));
AppendEndOfMembers();
}
else
{
//primitives like strings
_appendExpressions.Add(Expression.Call(SbArgExpression, memberAppendMethod, getMemberValue));
}
//my code ends here.
AppendQuotesIfRequiredForType(memberInfo);
}
此错误与以下异常有关：
This errors with the following exception:
System.ArgumentException: "Method System.String Join[T](System.String, System.Collections.Generic.IEnumerable`1[T]) is a generic method definition"
at System.Linq.Expressions.Expression.ValidateMethodInfo(MethodInfo method)
at System.Linq.Expressions.Expression.ValidateMethodAndGetParameters(Expression instance, MethodInfo method)
at System.Linq.Expressions.Expression.Call(MethodInfo method, Expression arg0, Expression arg1)
at MyNameSpace.Common.ToStringBuilder`1.AppendMember(MemberInfo memberInfo) in C:\myproject\MyNamespace.Common\ToStringBuilder.cs:line 206
我开始搜索该错误消息，发现人们在谈论使用Expression.Lamba
I started googling that error message and found people talking about using Expression.Lamba() to wrap calls to generic methods, at which point I realised I was way out of my depth.
因此，假设我有一个List mylist，我如何生成一个Expression如上所示这将等效于string.Join（，，mylist）; ？
So, assuming I have a List mylist, how do I generate an Expression as above that will do the equivalent of string.Join(", ", mylist); ?
感谢！
推荐答案
你的通用列表（T类型），你可以做
To get the "generic type" of your generic list (the type of "T"), you can do
var genericListType= type.GetGenericArguments()[0];
所以在你的elseif中，你可以做到（可能更容易，
so in your elseif, you could do (it might be easier, I just stay as close as possible to your code)
else if (type.IsGenericType && (type.GetGenericTypeDefinition() == typeof(List<>)))
{
// now to emit some code to do the below, you wouldn't think it'd be this hard...
// string.Join(", ", genericList);
AppendStartOfMembers();
//this returns null, because generics are not well supported by the reflection API, boo!
var stringJoinMethod = typeof(string).GetGenericMethod("Join", new[] { typeof(string), typeof(IEnumerable<>) });
var CommaSpace = Expression.Constant(", ");
var genericListType= type.GetGenericArguments()[0];
var genericStringJoinMethod = stringJoinMethod.MakeGenericMethod(new[]{genericListType});
// this doesn't work, throws an ArgumentException as below
getMemberValue = Expression.Call(genericStringJoinMethod , CommaSpace, getMemberValue);
_appendExpressions.Add(Expression.Call(SbArgExpression, memberAppendMethod, getMemberValue));
AppendEndOfMembers();
}
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