创建通用类型的实例

Question

I want to build a generic Observable Collection, that load values from the database to its items. In order to assign the values to the property of the item, I want to create an instance of the object. However, I get an error for the Activator.CreateInstance Command: "'T' is a type, which is not valid in the given context"

public class ListBase<T> : ObservableCollection<T>
{
    private DbTable tab;


    public ListBase()
    {
        tab = DbCatalog.Tables.Where(x => x.ModelObjectType == typeof(T).Name).First();

        LoadValues();
    }

    private void LoadValues()
    {

        foreach (DataRow r in tab.GetValues.Rows)
        {
            T o = (T)Activator.CreateInstance(T); //<-- The (T) at the end throws the error

            var p = o.GetType().GetProperty("xyz_property");

            if (p.PropertyType == typeof(int))
            {
                p.SetValue(o, Convert.ToInt32(r["xyz_fromDB"]));
            }

        }
    }

}

Answer 1

There's no need to use Activator.CreateInstance , the correct way to do this is to new it up like you would any object - but this requires a new constraint :

public class ListBase<T> : ObservableCollection<T> where T : new()
{

}

And now you can simply do this:

T o = new T();

Answer 2

You should use: CreateInstance(typeof(T)) , typeof returns an object of class System.Type which will work.

There is a difference in C# between a 'Generic' type T and an instance of System.Type . Activator.CreateInstance requires the latter.

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Edit: You should generally use DavidG's method , it is cleaner. You can use the Activator when:

• You can't add a generic constraint for some reason.
• You want to pass arguments to the constructor. (the new() constraint implies a parameterless constructor, with the Activator you can pass parameters.)