有没有人可以帮助我这个例外？ [英] could anyone give me a help in this exception ?

问题描述

大家好，

我在一个项目上工作，我使用devexpress mvc gridview&我试图分组行

这里是我的代码

hi everyone,
iam working on a project where iam using devexpress mvc gridview & iam trying to group rows
here's my code

public static object Sum(this IQueryable query, string fieldName)
        {
            if (query.Count() == 0)
                return 0;

            var parameter = Expression.Parameter(query.ElementType, string.Empty);
            var propertyInfo = query.ElementType.GetProperty(fieldName);
            var propertyAccess = Expression.MakeMemberAccess(parameter, propertyInfo);
            var propertyAccessExpression = Expression.Lambda(propertyAccess, parameter);
            var expression = Expression.Call(
                typeof(Queryable),
                "Sum",
                new Type[] { query.ElementType },
                query.Expression,
                Expression.Quote(propertyAccessExpression)
            );
            return query.Provider.Execute(expression);
        }

但这里出现错误：

but here the error i got :

No generic method 'Sum' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

PLZ任何人都有相同的情况并且得到解决方案会告诉我有什么问题



提前感谢

plz anyone have the same situation &got a solution would tell me what's the wrong

thanks in advance

推荐答案

fieldName 不代表具有支持类型之一的属性：

• int / Nullable< int>
• long / Nullable< long> ;
• float / Nullable< float>
• double / Nullable< double>
• decimal / Nullable< decimal>

The fieldName you've passed in doesn't represent a property with one of the supported types:

• int / Nullable<int>
• long / Nullable<long>
• float / Nullable<float>
• double / Nullable<double>
• decimal / Nullable<decimal>

var parameter = Expression.Parameter(query.ElementType, string.Empty);
var propertyInfo = query.ElementType.GetProperty(fieldName);
var propertyAccess = Expression.MakeMemberAccess(parameter, propertyInfo);

Expression body;
Type propertyType = propertyAccess.Type;
Type baseType = Nullable.GetUnderlyingType(propertyType); // Returns null if the type is not Nullable<T>

switch (Type.GetTypeCode(baseType ?? propertyType))
{
    case TypeCode.Int32:
    case TypeCode.Int64:
    case TypeCode.Single:
    case TypeCode.Double:
    case TypeCode.Decimal:
    {
        // Supported type - use the property directly:
        body = propertyAccess;
        break;
    }
    case TypeCode.Byte:
    case TypeCode.SByte:
    case TypeCode.Int16:
    case TypeCode.UInt16:
    {
        // Small enough to fit in an Int32:
        Type targetType = typeof(int);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
    case TypeCode.UInt32:
    {
        // Convert to an Int64:
        Type targetType = typeof(long);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
    default:
    {
        // Unknown type - try converting to Double.
        // If there is no conversion, this will throw an exception.
        
        Type targetType = typeof(double);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
}

var propertyAccessExpression = Expression.Lambda(body, parameter);

var expression = Expression.Call(
    typeof(Queryable),
    "Sum",
    new Type[] { query.ElementType },
    query.Expression,
    Expression.Quote(propertyAccessExpression)
);

return query.Provider.Execute(expression);

此外，还有没有内置的 Count 非泛型 IQueryable 界面的扩展方法。除非您已创建自己的版本或导入第三方版本，否则您方法的第一行将无效。

Also, there is no built-in Count extension method for the non-generic IQueryable interface. Unless you've created your own, or imported a third-party version, the first line of your method won't work.

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