对于具有对象键的基本类型，IQueryable GroupBy Lambda表达式失败(IQueryable GroupBy Lambda expression fails for primitive

对于具有对象键的基本类型，IQueryable GroupBy Lambda表达式失败(IQueryable GroupBy Lambda expression fails for primitive types with object key)

我正在为IQueryable<TSource>创建Lambda Expression，以下是我的扩展方法代码，我需要调用它：

queryableData.GroupBy<int,Product>("ID")

queryableData.GroupBy<string,Product>("Name")

public static IQueryable<IGrouping<TKey,TSource>> GroupBy<TKey,TSource>(this IQueryable<TSource> queryable, string propertyName) { // Access the propertyInfo, using Queryable Element Type (Make it Case insensitive) var propInfo = queryable.ElementType.GetProperty(propertyName,BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance); // Access the Collection / Queryable Type var collectionType = queryable.ElementType; // Creating Group Parameter Expression var groupParameterExpression = Expression.Parameter(collectionType, "g"); // Create MemberEXpression with the Property (access the property of a Type) var propertyAccess = Expression.MakeMemberAccess(groupParameterExpression, propInfo); // Create Lambda Expression var lambdaExpression = Expression.Lambda<Func<TSource,TKey>>(propertyAccess, groupParameterExpression); // Return GroupBy result return queryable.GroupBy(lambdaExpression); }

我的目标是生成Expression<Func<TSource,object>>而不是Expression<Func<TSource,TKey>> ，这样可以在不提供Key类型的情况下调用它，以下是代码：

public static IQueryable<IGrouping<object, TSource>> GroupByObjectKey<TSource>(this IQueryable<TSource> queryable, string propertyName) { // Access the propertyInfo, using Queryable Element Type (Make it Case insensitive) var propInfo = queryable.ElementType.GetProperty(propertyName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance); // Access the Collection / Queryable Type var collectionType = queryable.ElementType; // Creating Group Parameter Expression var groupParameterExpression = Expression.Parameter(collectionType, "g"); // Create MemberEXpression with the Property (access the property of a Type) var propertyAccess = Expression.MakeMemberAccess(groupParameterExpression, propInfo); // Create Lambda Expression var lambdaExpression = Expression.Lambda<Func<TSource, object>>(propertyAccess, groupParameterExpression); // Return GroupBy result return queryable.GroupBy(lambdaExpression); }

现在我能够使其适用于字符串类型，如下所示：

queryableData.GroupBy<Product>("Name")

但它跟随调用整数类型失败，异常如下所述：

queryableData.GroupBy<Product>("Id")

Expression of type 'System.Int32' cannot be used for return type 'System.Object'

这是类型转换的一个明显案例，但我很惊讶为什么一个类型会拒绝转换为Object基类，可能是什么原因，任何指针/建议

I am in process of creating Lambda Expression for an IQueryable<TSource>, following is my extension method code, which I need to call like:

queryableData.GroupBy<int,Product>("ID")

queryableData.GroupBy<string,Product>("Name")

public static IQueryable<IGrouping<TKey,TSource>> GroupBy<TKey,TSource>(this IQueryable<TSource> queryable, string propertyName) { // Access the propertyInfo, using Queryable Element Type (Make it Case insensitive) var propInfo = queryable.ElementType.GetProperty(propertyName,BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance); // Access the Collection / Queryable Type var collectionType = queryable.ElementType; // Creating Group Parameter Expression var groupParameterExpression = Expression.Parameter(collectionType, "g"); // Create MemberEXpression with the Property (access the property of a Type) var propertyAccess = Expression.MakeMemberAccess(groupParameterExpression, propInfo); // Create Lambda Expression var lambdaExpression = Expression.Lambda<Func<TSource,TKey>>(propertyAccess, groupParameterExpression); // Return GroupBy result return queryable.GroupBy(lambdaExpression); }

My aim is to generate Expression<Func<TSource,object>> instead of Expression<Func<TSource,TKey>>, so that it can be called without providing the Key type, following is code:

public static IQueryable<IGrouping<object, TSource>> GroupByObjectKey<TSource>(this IQueryable<TSource> queryable, string propertyName) { // Access the propertyInfo, using Queryable Element Type (Make it Case insensitive) var propInfo = queryable.ElementType.GetProperty(propertyName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance); // Access the Collection / Queryable Type var collectionType = queryable.ElementType; // Creating Group Parameter Expression var groupParameterExpression = Expression.Parameter(collectionType, "g"); // Create MemberEXpression with the Property (access the property of a Type) var propertyAccess = Expression.MakeMemberAccess(groupParameterExpression, propInfo); // Create Lambda Expression var lambdaExpression = Expression.Lambda<Func<TSource, object>>(propertyAccess, groupParameterExpression); // Return GroupBy result return queryable.GroupBy(lambdaExpression); }

Now I able to make it work for string type as follows:

queryableData.GroupBy<Product>("Name")

but it fails on following call for integer type with exception as stated underneath:

queryableData.GroupBy<Product>("Id")

Expression of type 'System.Int32' cannot be used for return type 'System.Object'

This is a clear case of Type conversion, but I am surprised that why a type would refuse to be converted to Object base class, what could be the rationale, any pointer / suggestion

最满意答案

正如评论已经指出的那样，您需要向object添加转换：

var convertToObject = Expression.Convert(propertyAccess, typeof(object)); var lambdaExpression = Expression.Lambda<Func<TSource, object>>(convertToObject, groupParameterExpression);

As comments already pointed out, you need to add a conversion to object:

var convertToObject = Expression.Convert(propertyAccess, typeof(object)); var lambdaExpression = Expression.Lambda<Func<TSource, object>>(convertToObject, groupParameterExpression);