Why is this an issue?

An IDisposable object should be disposed (there are some rare exceptions where not disposing is fine, most notably Task). If a class has an IDisposable field, there can be two situations:

• The class observes a field that is under the responsibility of another class.
• The class owns the field, and is therefore responsible for calling Dispose on it.

In the second case, the safest way for the class to ensure Dispose is called is to call it in its own Dispose function, and therefore to be itself IDisposable. A class is considered to own an IDisposable field resource if it created the object referenced by the field.

Noncompliant code example

public class ResourceHolder   // Noncompliant; doesn't implement IDisposable
{
  private FileStream fs;  // This member is never Disposed
  public void OpenResource(string path)
  {
    this.fs = new FileStream(path, FileMode.Open); // I create the FileStream, I'm owning it
  }
  public void CloseResource()
  {
    this.fs.Close();
  }
}

Compliant solution

public class ResourceHolder : IDisposable
{
  private FileStream fs;
  public void OpenResource(string path)
  {
    this.fs = new FileStream(path, FileMode.Open); // I create the FileStream, I'm owning it
  }
  public void CloseResource()
  {
    this.fs.Close();
  }

  public void Dispose()
  {
    this.fs.Dispose();
  }
}

Resources

• MITRE, CWE-459 - Incomplete Cleanup