By using this function you can get Iworkspace using IFeatureClass.

public IWorkspace getWorkSpace(IFeatureClass fetCls)
 {
 IWorkspace res = null;
 try
 {
 IFeatureDataset pFeatDataSet = fetCls.FeatureDataset;
 IDataset pDataSet = pFeatDataSet as IDataset;
 res = pDataSet.Workspace;
 }
 catch (Exception ex)
 {
 return null;
}
return res;
}