我希望能够编写一个通用表达式，用户可以用它来描述他想要如何在一系列类型中进行转换。
表达式可能类似于：
Expression<Func<PlaceHolder,object>> sample =
x=> (object)EqualityComparer<PlaceHolder>.GetHashCode(x)
我想将其转换为::
Expression<Func<Foo,object>> sample =
x=> (object)EqualityComparer<Foo>.GetHashCode(x)
我可以访问表达式，并将PlaceHolder参数替换为x，但是我无法解析泛型类型调用。
表达式是用户提供的，您无法为表达式分配通用方法。
最终结果总是返回一个对象，表达式始终来自T=>object。我将为默认规则将替换的任何对象编译一个新表达式。
这是我现有的代码，但它似乎非常复杂。
// ReSharper disable once InconsistentNaming
// By design this is supposed to look like a generic parameter.
public enum TEnum : long
{
}
internal sealed class EnumReplacer : ExpressionVisitor
{
private Type ReplacePlaceHolder(Type type)
{
if (type.IsByRef)
{
return ReplacePlaceHolder(type.GetElementType()).MakeByRefType();
}
if (type.IsArray)
{
// expressionTrees can only deal with 1d arrays.
return ReplacePlaceHolder(type.GetElementType()).MakeArrayType();
}
if (type.IsGenericType)
{
var typeDef = type.GetGenericTypeDefinition();
var args = Array.ConvertAll(type.GetGenericArguments(), t => ReplacePlaceHolder(t));
return typeDef.MakeGenericType(args);
}
if (type == typeof(TEnum))
{
return _enumParam.Type;
}
return type;
}
private MethodBase ReplacePlaceHolder(MethodBase method)
{
var newCandidate = method;
var currentParams = method.IsGenericMethod ? ((MethodInfo)method).GetGenericMethodDefinition().GetParameters() : method.GetParameters();
// ReSharper disable once PossibleNullReferenceException
if (method.DeclaringType.IsGenericType)
{
var newType = ReplacePlaceHolder(method.DeclaringType);
var methodCandidates = newType.GetMembers()
.OfType<MethodBase>()
.Where(x => x.Name == method.Name
&& x.IsStatic == method.IsStatic
&& x.IsGenericMethod == method.IsGenericMethod).ToArray();
// grab the first method that wins. Not 100% correct, but close enough.
// yes an evil person could define a class like this::
// class C<T>{
// public object Foo<T>(T b){return null;}
// public object Foo(PlaceHolderEnum b){return new object();}
// }
// my code would prefer the former, where as C#6 likes the later.
newCandidate = methodCandidates.First(m => TestParameters(m, currentParams));
}
if (method.IsGenericMethod)
{
var genericArgs = method.GetGenericArguments();
genericArgs = Array.ConvertAll(genericArgs, temp => ReplacePlaceHolder(temp));
newCandidate = ((MethodInfo)newCandidate).GetGenericMethodDefinition().MakeGenericMethod(genericArgs);
}
return newCandidate;
}
private Expression ReplacePlaceHolder(MethodBase method, Expression target, ReadOnlyCollection<Expression> arguments)
{
// no point in not doing this.
var newArgs = Visit(arguments);
if (target != null)
{
target = Visit(target);
}
var newCandidate = ReplacePlaceHolder(method);
MethodInfo info = newCandidate as MethodInfo;
if (info != null)
{
return Expression.Call(target, info, newArgs);
}
return Expression.New((ConstructorInfo)newCandidate, newArgs);
}
private bool TestParameters(MethodBase candidate, ParameterInfo[] currentParams)
{
var candidateParams = candidate.GetParameters();
if (candidateParams.Length != currentParams.Length) return false;
for (int i = 0; i < currentParams.Length; i++)
{
// the names should match.
if (currentParams[i].Name != candidateParams[i].Name) return false;
var curType = currentParams[i].ParameterType;
var candidateType = candidateParams[i].ParameterType;
// Either they are the same generic type arg, or they are the same type after replacements.
if (!((curType.IsGenericParameter &&
curType.GenericParameterPosition == candidateType.GenericParameterPosition)
|| ReplacePlaceHolder(curType) == candidateType))
{
return false;
}
}
return true;
}
private readonly ParameterExpression _enumParam;
public EnumReplacer(ParameterExpression enumParam)
{
_enumParam = enumParam;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (node.Type == typeof(TEnum))
{
return _enumParam;
}
if (node.Type == typeof(TypeCode))
{
return Expression.Constant(Type.GetTypeCode(_enumParam.Type));
}
return base.VisitParameter(node);
}
protected override Expression VisitUnary(UnaryExpression node)
{
if (node.NodeType == ExpressionType.Convert || node.NodeType == ExpressionType.ConvertChecked)
{
var t = ReplacePlaceHolder(node.Type);
// this isn't perfect. The compiler loves inserting random casts. To be protective and offer the most range, TEnum should be a long.
var method = node.Method == null ? null : ReplacePlaceHolder(node.Method);
return node.NodeType == ExpressionType.ConvertChecked
? Expression.ConvertChecked(Visit(node.Operand), t, (MethodInfo) method)
: Expression.Convert(Visit(node.Operand), t, (MethodInfo) method);
}
if (node.Operand.Type == typeof(TEnum))
{
var operand = Visit(node.Operand);
return node.Update(operand);
}
return base.VisitUnary(node);
}
private MemberInfo ReplacePlaceHolder(MemberInfo member)
{
if (member.MemberType == MemberTypes.Method || member.MemberType == MemberTypes.Constructor)
{
return ReplacePlaceHolder((MethodBase) member);
}
var newType = ReplacePlaceHolder(member.DeclaringType);
var newMember = newType.GetMembers().First(x => x.Name == member.Name);
return newMember;
}
protected override Expression VisitNewArray(NewArrayExpression node)
{
var children = Visit(node.Expressions);
// Despite returning T[], it expects T.
var type = ReplacePlaceHolder(node.Type.GetElementType());
return Expression.NewArrayInit(type, children);
}
protected override MemberMemberBinding VisitMemberMemberBinding(MemberMemberBinding node)
{
var newMember = ReplacePlaceHolder(node.Member);
var bindings = node.Bindings.Select(x => VisitMemberBinding(x));
return Expression.MemberBind(newMember, bindings);
}
protected override MemberListBinding VisitMemberListBinding(MemberListBinding node)
{
var prop = ReplacePlaceHolder(node.Member);
var inits = node.Initializers.Select(x => VisitElementInit(x));
return Expression.ListBind(prop, inits);
}
protected override Expression VisitMethodCall(MethodCallExpression node)
{
return ReplacePlaceHolder(node.Method, node.Object, node.Arguments);
}
protected override MemberAssignment VisitMemberAssignment(MemberAssignment node)
{
var expr = Visit(node.Expression);
var prop = ReplacePlaceHolder(node.Member);
return Expression.Bind(prop, expr);
}
protected override ElementInit VisitElementInit(ElementInit node)
{
var method = ReplacePlaceHolder(node.AddMethod);
var args = Visit(node.Arguments);
return Expression.ElementInit((MethodInfo)method, args);
}
protected override Expression VisitNew(NewExpression node)
{
return ReplacePlaceHolder(node.Constructor, null, node.Arguments);
}
protected override Expression VisitConstant(ConstantExpression node)
{
// replace typeof expression
if (node.Type == typeof(Type) && (Type)node.Value == typeof(TEnum))
{
return Expression.Constant(_enumParam.Type);
}
// explicit usage of default(TEnum) or (TEnum)456
if (node.Type == typeof(TEnum))
{
return Expression.Constant(Enum.ToObject(_enumParam.Type, node.Value));
}
return base.VisitConstant(node);
}
}
用法就像这样::
class Program
{
public class Holder
{
public int Foo { get; set; }
}
public class Foo<T1,T2> : IEnumerable
{
public object GenericMethod<TM, TM2>(TM2 blarg) => blarg.ToString();
public IList<Foo<T1, T2>> T { get; set; } = new List<Foo<T1, T2>>();
public T1 Prop { get; set; }
public void Add(int i) { }
public Holder Holder { get; set; } = new Holder {};
public IEnumerator GetEnumerator()
{
throw new NotImplementedException();
}
}
public enum LongEnum:ulong
{
}
static void Main(string[] args)
{
Expression<Func<TEnum, TypeCode, object>> evilTest = (x,t) =>
TypeCode.UInt64 == t
? (object)new Dictionary<TEnum, TypeCode>().TryGetValue(checked((x - 407)), out t)
: new Foo<string, TEnum> { Holder = {Foo =6}, T = new []
{
new Foo<string, TEnum>
{
T = {
new Foo<string, TEnum>{1,2,3,4,5,6,7,8,9,10,11,12}
}
},
new Foo<string, TEnum>
{
Prop = $"What up hello? {args}"
}
}}.GenericMethod<string, TEnum>(x);
Console.WriteLine(evilTest);
var p = Expression.Parameter(typeof(LongEnum), "long");
var expressionBody = new EnumReplacer(p).Visit(evilTest.Body);
var q = Expression.Lambda<Func<LongEnum, object>>(expressionBody, p);
var func =q.Compile();
var res = func.Invoke((LongEnum)1234567890123Ul);
答案 0 :(得分：1)
修改现有的表达式树只是为了改变其中使用的类型，这似乎是一个傻瓜差事。如果您尝试对该类型的对象进行操作，那么您将遇到更多问题。
但是你为什么要让所有人都改变现有的树？您正在尝试参数化一个需要泛型的类型。只需创建一个返回所需表达式的泛型方法（类型为您的参数）。
Expression<Func<T, object>> CreateConverter<T>() =>
(T x) => EqualityComparer<T>.Default.GetHashCode(x);
无需创建虚假占位符类型，泛型类型参数 是占位符。
如果您需要将其设置为可插拔，请将该方法放在界面中，用户将提供执行转换的实现。