public sealed class FeatureDatasetDefinition : Definition, System.IDisposable  

public async Task ObtainingDefinitionFromGeodatabase()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
    {
      // Remember that for Enterprise databases you have to qualify your dataset names with the DatabaseName and UserName.
      TableDefinition enterpriseTableDefinition = geodatabase.GetDefinition<TableDefinition>("LocalGovernment.GDB.CitizenContactInfo");

      // It does not matter if the dataset is within a FeatureDataset or not.
      FeatureClassDefinition featureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>("LocalGovernment.GDB.FireStation");

      // GetDefinition For a RelationshipClass.
      RelationshipClassDefinition relationshipClassDefinition = geodatabase.GetDefinition<RelationshipClassDefinition>("LocalGovernment.GDB.AddressPointHasSiteAddresses");

      // GetDefinition For a FeatureDataset.
      FeatureDatasetDefinition featureDatasetDefinition = geodatabase.GetDefinition<FeatureDatasetDefinition>("LocalGovernment.GDB.Address");
    }
  });
}

public async Task ObtainingDefinitionsFromGeodatabase()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(
      new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
    {
      var enterpriseDefinitions = 
        geodatabase.GetDefinitions<FeatureClassDefinition>();
      IEnumerable<Definition> featureClassesHavingGlobalID = 
        enterpriseDefinitions.Where(definition => definition.HasGlobalID());

      var featureDatasetDefinitions = 
        geodatabase.GetDefinitions<FeatureDatasetDefinition>();
      bool electionRelatedFeatureDatasets = 
        featureDatasetDefinitions.Any(
          definition => definition.GetName().Contains("Election"));

      var attributedRelationshipClassDefinitions = 
        geodatabase.GetDefinitions<AttributedRelationshipClassDefinition>();

      var relationshipClassDefinitions = 
        geodatabase.GetDefinitions<RelationshipClassDefinition>();
    }
  });
}

// Deleting a FeatureDataset named as 'Parcel_Information'

FeatureDatasetDefinition featureDatasetDefinition = geodatabase.GetDefinition<FeatureDatasetDefinition>("Parcel_Information");
FeatureDatasetDescription featureDatasetDescription = 
  new FeatureDatasetDescription(featureDatasetDefinition);

SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

// Delete an existing feature dataset named as 'Parcel_Information'
schemaBuilder.Delete(featureDatasetDescription);
schemaBuilder.Build();