c# best way to declare a class-level value in base class and set it at derived classes
What I've guessed from your code (and I might be wrong) is that you need to get some string from a type used in a persistence layer. The string might be a table name, or a value for a certain column. I've decided to call this a discriminator. Multiple types can have the same discriminator but each type can only have one.

I think it makes sense to introduce an inheritable attribute here, let's do that:

[AttributeUsage(AttributeTargets.Class, Inherited = true, AllowMultiple = false)]
sealed class DiscriminatorAttribute : Attribute
{
    readonly string discriminator;

    public DiscriminatorAttribute(string discriminator) 
   { 
        this.discriminator = discriminator;
   }

    public string Discriminator
    {
        get { return discriminator; }
    }
}


We'll also add a class which takes a type and gets us our string (if we have one).

public static class DiscriminatorService
{
    public static string LookupDiscriminatorForType(Type t)
    {
        var result = (from a in t.GetCustomAttributes(true)
                 where a is DiscriminatorAttribute
                 select a).SingleOrDefault() as DiscriminatorAttribute;
        return result != null ? result.Discriminator : null;
    }
}


This will allow you to change your repository to the following:

public class Repository&lt;T&gt; where T : CoreEntity
{
    public void DoSomething()
    {
        var objType = DiscriminatorService.LookupDiscriminatorForType(typeof(T));
        Console.WriteLine("Object Type for: " + typeof(T).Name  + " is " + objType);
    }
}


Some important things to note:


You no longer need to create an instance to get the string
you can remove the new() generic constraint


Obviously you need to decorate your classes with the new attribute:

[Discriminator("DEPARTMAN")]
public class DepartmanBase : CoreEntity
{
    public string DepartmanName { get; set; }
    public int ManagerId { get; set; }
}
public class Departman : DepartmanBase
{
    // Inherits discriminator from DepartmanBase
}

public class DepartmanView : DepartmanBase
{
    // Inherits discriminator from DepartmanBase
    public string ManagerName { get; set; }
}

[Discriminator("PERSONEL")]
public class Personnel : CoreEntity
{
    public string PersonnelName { get; set; }
}


Here's the full code in .Net Fiddle