Question

我有数据库表。它的ORM是：

public long Id { get; set; }
public System.Guid AttrId { get; set; }
public System.Guid ProdId { get; set; }
public string Value { get; set; }

public virtual Attributes Attributes { get; set; }
public virtual Products Products { get; set; }

正如您所看到的，value是一种字符串类型。我试图通过此字段获取最小值和最大值（某些值表示为double）。所以这是我的方法：

public double[] GetMaxMinVals(Guid attrId)
{
    double[] res = new double[2];
    using(entityContext = new SiteDBEntities())
    {
        res[0] = entityContext.ProductAttributes.Where(x=>x.AttrId == attrId)
            .Min(x => Convert.ToDouble(x.Value));
        res[1] = entityContext.ProductAttributes.Where(x => x.AttrId == attrId)
            .Max(x => Convert.ToDouble(x.Value));
    }

    return res;
}

但我得到例外：

LINQ to Entities无法识别方法'Double ToDouble（System.String）'方法，并且此方法无法转换为商店表达式。

那么如何搜索像小数一样的字符串值？

Answer 1

此处的问题是您的查询将被翻译为SQL并在数据库上运行，而Entity Framework不知道如何将Convert.ToDouble转换为有效的SQL代码。

因此您可以转发到double，如下所示，稍后会转换为SQL CAST AS语句。

res[0] = entityContext.ProductAttributes.Where(x=>x.AttrId == attrId).Min(x => (double)x.Value);
res[1] = entityContext.ProductAttributes.Where(x => x.AttrId == attrId).Max(x => (double)x.Value);

Answer 2

首先，您可以将Value转换为双倍，然后使用Max/Min：

public double[] GetMaxMinVals(Guid attrId)
    {
        double[] res = new double[2];
        using(entityContext = new SiteDBEntities())
        {
            res[0] = entityContext.ProductAttributes
              .Where(x => x.AttrId == attrId)
              .Select(x => x.Value)
              .Cast<double>()
              .Min();
        }

        return res;
    }

Answer 3

在EF Dbcontext中不支持“Convert.ToDouble”，你可以修复它：

    $Graduates = "45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,73,74,75,76,84,85,86,87,88,89,91,92,93,94,95,96,98,99,100,101,103,104,105,106,107,108,109,110,111,112,113,114,117,122,123,126,127,131,135,136,137,138,139,140,142,143,146,156"; //MajorID

    $tableName = $SemYearSettings->getYearSem($Enroll);

    $QryFirstNursing = "SELECT *
            FROM {$tableName}
            WHERE {$tableName}.MajorID NOT IN ('{$Graduates}')
            AND {$tableName}.Category = {$CategoryIDN}";        

    $resultFirstNursing = openQry($QryFirstNursing);

实体框架中字符串的MIN和MAX值

3 个答案: