Add to List Property using the properties Index C#

If you can't change the class definition, your main alternative option is to use reflection.

void Main()
{
  var bus = new Bus();
  var data = new string[6] { "A", "B", "C", "D", "E", "F" };

  for (var i = 1; i <= 6; i++)
  {
    bus.GetType().GetProperty("Val" + i.ToString()).SetValue(bus, data[i - 1]);
  }

  Console.WriteLine(bus.Val5); // E
}

public class Bus 
{
  public string Val1 {get;set;}
  public string Val2 {get;set;}
  public string Val3 {get;set;}
  public string Val4 {get;set;}
  public string Val5 {get;set;}
  public string Val6 {get;set;}
}

Needless to say, this is quite expensive, and may be hard to maintain. Make sure you don't have a more reasonable option (e.g. changing the class to contain arrays instead of indexed properties, using code generation...) before using this.

Even if your database has some COBOL-like monstrosity with 150 indexed columns, there shouldn't be a reason why your application can't deal with them in the form of Item[34] instead of Item34 - isolate the application code from the fixed constraints you're not happy with.

Why not use

public class Bus
{
    public string[] Val = new string[127];
}

j = 0;
for (int i = 0; i<lines.Length; i++)
{
    foreach(Bus BusProp in BusList)
    {
        BusProp.Val[j] = line[i + j];
        j =+ 1;
    }
}

Try this

var typ = typeof(Bus);

var prop = typ.GetProperty($"Val{j}");

I feel like the answers so far haven't satisfied your needs, thus here's my solution:

    static void Main(string[] args)
    {
        //Create a string array containing the desired property names, in this case I'll use a loop
        List<string> DesiredProperties = new List<string>(); 

        for (int i = 0; i < 100; i++)
        {
            DesiredProperties.Add(string.Format("Property{0}", i));
        }

        //Call the method that returns the object and pass the array as parameter
        var Bus = CreateDynamicObject(DesiredProperties);

        //Display one of the properties
        Console.WriteLine(Bus.Property99);
        Console.Read();
    }
    private static dynamic CreateDynamicObject(List<string> PropertyList)
    {
        dynamic obj = new System.Dynamic.ExpandoObject();
        foreach (string Prop in PropertyList)
        {
            //You can add the properties using a dictionary. You can also give them an initial value
            var dict = (IDictionary<string, object>)obj;
            dict.Add(Prop, string.Format("The value of {0}", Prop));
        }
        return obj;
    }

This piece of code will add 100 properties to var "Bus", which can be accessed and applied a value at will.