Passing an empty array as default value of an optional parameter in C#

We can't create compile-time constants of object references.

The only valid compile-time constant for reference types you can use is null, so change your code to this:

public void DoSomething(int index, string[] array = null)

And inside your method do this:

array = array ?? new ushort[0];

The documentation for optional arguments [https://msdn.microsoft.com/en-us/library/dd264739.aspx] says:

Each optional parameter has a default value as part of its definition. A default value must be one of the following types of expressions:

• a constant expression;
• an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;
• an expression of the form default(ValType), where ValType is a value type.

Since new string[0] is neither a constant expression nor a new statement followed by a value type, it cannot be used as a default argument value.

You can try with null following

private Dictionary<string, string> LoadTagList(string[] skipTags = null)
{
  IRepository etRepo = new GenericRepository(_factory.ContextFactory);
  if (skipTags == null)
    return etRepo.Get().ToDictionary(et => et.Tag_Name, et => "");
  else
    return etRepo.Get(filter: e => !skipTags.Contains(e.Tag_Name)).ToDictionary(et => et.Tag_Name, et => "");
}