Calling ToString() on an object should always return a string. Returning null instead contravenes the method’s implicit contract.

Noncompliant Code Example

public override string ToString ()
{
  if (this.collection.Count == 0)
  {
    return null; // Noncompliant
  }
  else
  {
    // ...
  }
}

Compliant Solution

public override string ToString ()
{
  if (this.collection.Count == 0)
  {
    return string.Empty;
  }
  else
  {
    // ...
  }
}