public T GetDefinition<T>( 
   string name
)
where T: Definition

public void IterateDatasetsFromAFeatureDataset(Geodatabase geodatabase, string featureDatasetName = "City", string featureClassInFeatureDataset = "Buildings")
{
  // Open a feature dataset
  using (FeatureDataset cityFeatureDataset = geodatabase.OpenDataset<FeatureDataset>(featureDatasetName))
  {
    // Get a feature class definition from a feature dataset
    FeatureClassDefinition buildingsFeatureClassDefinition = cityFeatureDataset.GetDefinition<FeatureClassDefinition>(featureClassInFeatureDataset);

    // Iterate dataset definition
    IReadOnlyList<FeatureClassDefinition> cityFeatureClassDefinitions = cityFeatureDataset.GetDefinitions<FeatureClassDefinition>();
    foreach (FeatureClassDefinition cityFeatureClassDefinition in cityFeatureClassDefinitions)
    {
      // Use feature class definition
    }
  }
}