MIN and MAX value by string in Entity Framework

Question

I have database table. The ORM for it is:

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; }

As you can see value is a string type. I'm trying to get min and max values by this field (some values represented as double). So here is my method for this:

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;
}

But I getting exception:

LINQ to Entities does not recognize the method 'Double ToDouble(System.String)' method, and this method cannot be translated into a store expression.

So how can I search for a string value like a decimal?

Hari Prasad · Accepted Answer · 2016-06-02 08:57:46Z

2

The problem here is your query will be translated into SQL and run on the database, and Entity Framework doesn't know how to translate Convert.ToDouble into valid SQL code.

So you could cast to double as below, which will be later converted to SQL CAST AS statement.

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);

answered Jun 2, 2016 at 8:57

Hari Prasad

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1 Comment

dantey89 Over a year ago

Thanx for explaining!

Dmitrii Bychenko · Accepted Answer · 2016-06-02 09:12:44Z

1

First you can cast the Value to double then use 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;
    }

edited Jun 2, 2016 at 9:12

Dmitrii Bychenko

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answered Jun 2, 2016 at 8:51

Burak Karakuş

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3 Comments

dantey89 Over a year ago

Thanks. Now i know another one EF funtion and where to use it ;-)

Burak Karakuş Over a year ago

Did it work? Did you try? Cause when I tried it after I wrote the answer, I realized that it wouldn't for your case. If it works, I probably missed something.

Burak Karakuş Over a year ago

Okay, I am more than glad then!

Linh Tuan · Accepted Answer · 2016-06-02 08:52:01Z

1

In EF Dbcontext don't support "Convert.ToDouble", you can fix same that:

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

            return res;
        }

answered Jun 2, 2016 at 8:52

Linh Tuan

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