是否可以创建一个将项添加到实体框架dbset的通用方法？

Question

I have not worked with Entity Framework or generics before and am having some difficulty reducing my code.

I am parsing a text file to load 10 lookup tables with data that could possibly change nightly. The text files have a heading for the "type" followed by a list of key/value sets. I have this working perfectly, but I would like to refactor the code to clean it up and would like to use generic methods to accomplish this so I can reduce the duplicated code.

I have gotten the parsing down to a generic method, but I have not been able to figure out how to get the entities added to the context in a generic way. I have to loop over a list of the entities for each type and add them to the context:

void Main()
{
    switch (line.ToUpper())
    {
        case "AREA":
        {
            List<Area> areaList = this.GetItems<Area>(file);

            foreach (var element in areaList)
            {
                if (context.Area.Find(element.Id) == null)
                {
                    context.Area.Add(element);
                }
            }

            break;
        }
        case "CATEGORY":
        {
            List<Category> categoryList = this.GetItems<Category>(file);

            foreach (var element in categoryList)
            {
                if (context.Category.Find(element.Id) == null)
                {
                    context.Category.Add(element);
                }
            }

            break;
        }
    }
}

private List<T> GetItems<T>(StreamReader reader) where T : ILookupData, new()
{
    string line;
    List<T> list = new List<T>();            

    while (reader.Peek() == ' ')
    {
        line = reader.ReadLine().TrimStart();
        string[] tokens = line.Split(new string[] { " - " }, 2, StringSplitOptions.None);

        T item = new T();
        item.Id = Convert.ToInt32(tokens[0]);
        item.Description = (string)tokens[1];

        list.Add(item);
    }

    return list;
}

Update : The above works fine, but cannot seem to get the context added.

I have tried a few different things, but seem to keep getting this error when I try to generic up the context:

The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type of method.

The last thing I tried was to add a generic GetDbSet to the context:

public DbSet<T> GetDbSet<T>() where T : class
{
    return this.Set<T>();
}

But I get the same error in my controller adding this code to the GetItems method:

using (MyContext context = new MyContext())
{
    var dbSet = context.GetDbSet<T>();
}

Answer 1

I have a generic repository in my current project. This is its add method:

public void Add<T>(T newItem) where T : class
{
    db.Set<T>().Add(newItem);
}

where db is the DbContext object itself. The where T : class fixes that error about reference types. Without it, you could pass any type in as T, including bool or struct , or any value type, which DbSet.Add() can't handle. The where specifies that T must be a class, which is a reference type, and therefore allowed.