Calling <code>toString</code> on an object should always return a string. Returning <code>null</code> instead contravenes the method's implicit contract.

<h2>Noncompliant Code Example</h2>
<pre>
public override string ToString () {
  if (this.collection.Count == 0) {
    return null; // Noncompliant
  } else {
    // ...
</pre>

<h2>Compliant Solution</h2>
<pre>
public override string ToString () {
  if (this.collection.Count == 0) {
    return "";
  } else {
    // ...
</pre>