How do I concatenate strings in Entity Framework Query?

问题

How do I concatenate strings in Entity Framework 4 I have a data from a column and I want to save as a string a comma separated string like "value1, value2, value3" Is there a method or an operator do do this in EF4? Example: lets say that I have two columns Fruit and Farms with the following values:

• Apples
• Bananas
• Strawberries

If I do like this

var dataSource = this.context
    .Farms
    .Select(f => new
        {
            f.Id, 
            Fruits = string.Join(", ", f.Fruits)
        });

Sure I will get this error

LINQ to Entities does not recognize the method 'System.String Join(System.String, System.Collections.Generic.IEnumerable`1[System.String])' method, and this method cannot be translated into a store expression.

Is there any solution to this?

回答1:

You have to execute the query before projecting. Otherwise EF tries to translate the Join method into SQL (and obviously fails).

var results = this.context
                  .Farms
                  .ToList()
                  .Select(f => new
                      {
                          f.Id, 
                          Fruits = string.Join(", ", f.Fruits)
                      });

回答2:

Took @Yakimych answer and thought would provide mine if someone needed:

using (myDBEntities db = new myDBEntities())
            {
                var results = db.Table
                    .ToList()
                    .Where(x => x.LastName.StartsWith("K"))
                    .Select(
                    x => new
                    {
                        x.ID,
                        Name = x.LastName + ", " + x.FirstName
                    }
                    );

                lstCoaches.DataValueField = "ID";
                lstCoaches.DataTextField = "Name";
                lstCoaches.DataSource = results;
                lstCoaches.DataBind();
                ListItem item = new ListItem
                {
                    Value = "0",
                    Text = "-- Make a Selection --"
                };
                lstCoaches.Items.Insert(0,item);
                lstCoaches.SelectedIndex = 0;
            }

来源：https://stackoverflow.com/questions/4095658/how-do-i-concatenate-strings-in-entity-framework-query