public void AddFeatureClass( 
   FeatureDatasetDescription featureDatasetDescription,
   FeatureClassDescription featureClassDescription
)

public void AddFeatureClassToFeatureDatasetSnippet(Geodatabase geodatabase)
{
    // Adding a FeatureClass with name 'Tax_Jurisdiction' into a FeatureDataset named as 'Parcels_Information'

    string featureDatasetName = "Parcels_Information";
    string featureClassNameToAdd = "Tax_Jurisdiction";

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

    FeatureClassDefinition featureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>(featureClassNameToAdd);
    FeatureClassDescription featureClassDescription = new FeatureClassDescription(featureClassDefinition);

    SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

    // Add the 'Tax_Jurisdiction' FeatureClass to the 'Parcels_Information' FeatureDataset 
    schemaBuilder.AddFeatureClass(featureDatasetDescription, featureClassDescription);
    bool addStatus = schemaBuilder.Build();

    if (!addStatus)
    {
        IReadOnlyList<string> errors = schemaBuilder.ErrorMessages;
    }
}