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);更多推荐
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