我有這樣的表達：Expression<Func<MyEntity, bool>> exp = x => x.FirstName == "Jonas";該表達式被傳輸到另一個不具有類型的應用程序MyEntity。為了能夠執行表達式，我嘗試將其中的類型替換為ExpandoObject使用ExpressionVistor. public class ReplaceToExpandoVisitor : ExpressionVisitor { ParameterExpression _parameter; private Type _targetType = typeof(ExpandoObject); public ReplaceToExpandoVisitor(ParameterExpression p2) { _parameter = p2; } protected override Expression VisitParameter(ParameterExpression node) { return _parameter; } protected override Expression VisitMember(MemberExpression node) { if (node.Member.MemberType != System.Reflection.MemberTypes.Property) throw new NotSupportedException(); var memberName = node.Member.Name; var propBinder = Binder.GetMember(CSharpBinderFlags.None, memberName, GetType(), new[] { CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null) }); var inner = Visit(node.Expression); // this is the right way, right? var exp2 = Expression.Dynamic(propBinder, typeof(object), inner); // I need to convert it right? Otherwise it will be of type object? var propGetExpression = Expression.Convert(exp2, node.Type); return propGetExpression; } }然而，該表達式在執行時返回 false。所以我想我沒有正確訪問 Expandoobject 中的“屬性”。有人可以解釋我做錯了什么嗎？
1 回答
翻閱古今
TA貢獻1780條經驗 獲得超5個贊
我認為你使用活頁夾使事情變得過于復雜。ExpandoObject實現IDictionary<string, object>接口，因此您可以替換x => x.FirstName == "Jonas"為x => x["FirstName"] == "Jonas"哪個應該更容易。另外，您必須重寫VisitLambda, 來修改類型參數，否則轉換將失敗。
這是示例代碼：
public class ReplaceToExpandoVisitor<TSource> : ExpressionVisitor
{
private static readonly PropertyInfo ItemProperty = typeof(IDictionary<string, object>).GetProperty("Item");
private readonly ParameterExpression _targetParameter = Expression.Parameter(typeof(ExpandoObject));
protected override Expression VisitLambda<T>(Expression<T> node)
{
var body = this.Visit(node.Body);
var parameters = node.Parameters.Select(this.Visit).Cast<ParameterExpression>();
return Expression.Lambda(body, parameters);
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (node.Type == typeof(TSource))
{
return this._targetParameter;
}
return node;
}
protected override Expression VisitMember(MemberExpression node)
{
if (node.Member.MemberType != MemberTypes.Property)
{
throw new NotSupportedException();
}
string memberName = node.Member.Name;
return Expression.Convert(
Expression.Property(
this.Visit(node.Expression),
ItemProperty,
Expression.Constant(memberName)
),
((PropertyInfo)node.Member).PropertyType
);
}
}
用法：
Expression<Func<MyEntity, bool>> exp = x => x.FirstName == "Jonas";
Expression<Func<ExpandoObject, bool>> exp2 = (Expression<Func<ExpandoObject, bool>>) new ReplaceToExpandoVisitor<MyEntity>().Visit(exp);
dynamic obj = new ExpandoObject();
obj.FirstName = "Jonas";
bool result = exp2.Compile().Invoke(obj);
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