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

{
  await 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");
    }
  });
}

{
  //Url examples for (federated) feature services
  //(by "ref" online):
  var url =
      @"https://sampleserver6.arcgisonline.com/arcgis/rest/services/SF311/FeatureServer";
  //(federated by ref on portal)
  //https://portal.example.com/server/rest/services/FeatureServiceName/FeatureServer");
  //(federated by value - Hosted - on portal)
  //https://portal.example.com/server/rest/services/Hosted/FeatureServiceName/FeatureServer");

  await QueuedTask.Run(() =>
  {
    var uri = new Uri(url, UriKind.Absolute);
    using (var fs_db = new ArcGIS.Core.Data.Geodatabase(new ServiceConnectionProperties(uri)))
    {
      //Assuming that id 0 is a FeatureClass for the given service
      FeatureClassDefinition featureClassDefinition =
          fs_db.GetDefinition<FeatureClassDefinition>("0");
      string shapeField = featureClassDefinition.GetShapeField();
      IReadOnlyList<Field> fields = featureClassDefinition.GetFields();

      //Assuming that id 1 is a Table for the given service
      TableDefinition tableDefinition =
            fs_db.GetDefinition<TableDefinition>("1");
      string objectIDField = tableDefinition.GetObjectIDField();
    }
  });
}

{
  //Must be called within QueuedTask.Run().
  bool TableExists(Geodatabase geodatabase, string tableName)
  {
    try
    {
      using TableDefinition tableDefinition =
        geodatabase.GetDefinition<TableDefinition>(tableName);
      return true;
    }
    catch
    {
      // GetDefinition throws an exception if the definition doesn't exist
      return false;
    }
  }
}

{
  //Must be called within QueuedTask.Run().
  bool FeatureClassExists(Geodatabase geodatabase, string featureClassName)
  {
    try
    {
      using FeatureClassDefinition featureClassDefinition =
        geodatabase.GetDefinition<FeatureClassDefinition>(featureClassName);
      return true;
    }
    catch
    {
      // GetDefinition throws an exception if the definition doesn't exist
      return false;
    }
  }
}