Issues with generic List<T>

It looks like you're trying to use generics to create lists of different types based on a condition. The error message you're seeing, "Implicitly-typed variables must be initialized," is because implicitly-typed variables in C# (those declared with var) need to have their type determined at compile-time based on the right side of their initialization. Since you're declaring obj without assigning any value to it, the compiler can't infer what type it is.

Let's correct a few things in your code. Firstly, it's important to note that you can't create a List of a type that is not known at compile time, without using reflection or other advanced techniques. Here's a way to restructure your code to eliminate the immediate error and start on a path to a more dynamic solution:

object obj = null;

if (a == 1) obj = Activator.CreateInstance(typeof(CareerFile1));
else if (a == 2) obj = Activator.CreateInstance(typeof(CareerFile2));
el se if (a == 3) obj = Activator.CreateInstance(typeof(CareerFile3));

// Now we have an object, but what we want is a list of that object type
// To achieve this we're going to use reflection
Type objType = obj?.GetType();
if (objType != null) {
    var listType = typeof(List<>).MakeGenericType(objType);
    var listInstance = Activator.CreateInstance(listType);
    // Now you'll have to cast this to an appropriate type or use reflection to work with listInstance
}

// Use the listInstance ...

In this code:

• We remove the re-declaration of obj.
• Correct the assignment operator (=) to equality operator (==) in conditions.
• Use the object created from Activator.CreateInstance to determine the type of the list we're creating.

However, please note that working with a non-generic list means you'll lose some type safety, and you'll have to use reflection or dynamic casting when working with items in the list. If you can create a common interface or base class for the CareerFile objects, you could create a List or List instead, which would be more in line with good OOP practices.

First note that you have:

(a = 1)

And that should be:

(a == 1)

See the difference?

Also, instead of:

if (a == 1 ) obj = Activator.CreateInstance(typeof(CareerFile1));
if (a == 2 ) obj = Activator.CreateInstance(typeof(CareerFile2));
if (a == 3 ) obj = Activator.CreateInstance(typeof(CareerFile3));

You can do:

Type[] CareerTypes = new Type[]
	{typeof(CareerFile1), typeof(CareerFile2), typeof(CareerFile3)};
.
.
.
obj = Activator.CreateInstance(CareerTypes[a-1]);

As in: https://dotnetfiddle.net/dtghHE

And if CareerFile1, CareerFile2 and CareerFile3 inherit from a base then you can use the base instead of object.

I am not sure what you need to do but Activator.CreateInstance is advanced stuff. There might ber an easier way to do what you need to do.

Here's an example of how you might resolve these issues:

object obj = null; // Declare obj as an object type

if (a == 1) 
    obj = Activator.CreateInstance(typeof(CareerFile1));
else if (a == 2) 
    obj = Activator.CreateInstance(typeof(CareerFile2));
else if (a == 3) 
    obj = Activator.CreateInstance(typeof(CareerFile3));

dbo_Bll aaa = new dbo_Bll();

// You might need to cast the returned object to a specific type
// based on the logic above before adding it to the list.
List