public void Modify( 
   FeatureClassDescription featureClassDescription
)

public void AddFieldsInFeatureClassSnippet(Geodatabase geodatabase)
{
    // Adding following fields to the 'Parcels' FeatureClass
    // Global ID
    // Parcel_ID
    // Tax_Code
    // Parcel_Address

    // The FeatureClass to add fields
    string featureClassName = "Parcels";

    FeatureClassDefinition originalFeatureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>(featureClassName);
    FeatureClassDescription originalFeatureClassDescription = new FeatureClassDescription(originalFeatureClassDefinition);

    // The four new fields to add on the 'Parcels' FeatureClass
    FieldDescription globalIdField = FieldDescription.CreateGlobalIDField();
    FieldDescription parcelIdDescription = new FieldDescription("Parcel_ID", FieldType.GUID);
    FieldDescription taxCodeDescription = FieldDescription.CreateIntegerField("Tax_Code");
    FieldDescription addressDescription = FieldDescription.CreateStringField("Parcel_Address", 150);

    List<FieldDescription> fieldsToAdd = new List<FieldDescription> { globalIdField, parcelIdDescription,
taxCodeDescription, addressDescription };

    // Add new fields on the new FieldDescription list
    List<FieldDescription> modifiedFieldDescriptions = new List<FieldDescription>(originalFeatureClassDescription.FieldDescriptions);
    modifiedFieldDescriptions.AddRange(fieldsToAdd);

    // The new FeatureClassDescription with additional fields
    FeatureClassDescription modifiedFeatureClassDescription = new FeatureClassDescription(originalFeatureClassDescription.Name,
      modifiedFieldDescriptions, originalFeatureClassDescription.ShapeDescription);

    SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

    // Update the 'Parcels' FeatureClass with newly added fields
    schemaBuilder.Modify(modifiedFeatureClassDescription);
    bool modifyStatus = schemaBuilder.Build();

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

    await QueuedTask.Run(() =>
    {
      using (var kg = GetKnowledgeGraph())
      {
        if (kg == null)
          return;

        var entity_name = "Fruit";

        //Domains are managed on the GDB objects...
        var fruit_fc = kg.OpenDataset<FeatureClass>(entity_name);
        var fruit_fc_def = fruit_fc.GetDefinition();

        var fieldFruitTypes = fruit_fc_def.GetFields()
              .FirstOrDefault(f => f.Name == "FruitTypes");
        var fieldShelfLife = fruit_fc_def.GetFields()
            .FirstOrDefault(f => f.Name == "ShelfLife");

        //Create a coded value domain and add it to a new field
        var fruit_cvd_desc = new CodedValueDomainDescription(
          "FruitTypes", FieldType.String, 
          new SortedList<object, string> {
                          { "A", "Apple" },
                          { "B", "Banana" },
                          { "C", "Coconut" }
          })  {
            SplitPolicy = SplitPolicy.Duplicate,
            MergePolicy = MergePolicy.DefaultValue
        };

        //Create a Range Domain and add the domain to a new field description also
        var shelf_life_rd_desc = new RangeDomainDescription(
                                      "ShelfLife", FieldType.Integer, 0, 14);

        var sb = new SchemaBuilder(kg);
        sb.Create(fruit_cvd_desc);
        sb.Create(shelf_life_rd_desc);

        //Create the new field descriptions that will be associated with the
        //"new" FruitTypes coded value domain and the ShelfLife range domain
        var fruit_types_fld = new ArcGIS.Core.Data.DDL.FieldDescription(
                                      "FruitTypes", FieldType.String);
        fruit_types_fld.SetDomainDescription(fruit_cvd_desc);

        //ShelfLife will use the range domain
        var shelf_life_fld = new ArcGIS.Core.Data.DDL.FieldDescription(
"ShelfLife", FieldType.Integer);
        shelf_life_fld.SetDomainDescription(shelf_life_rd_desc);

        //Add the descriptions to the list of field descriptions for the
        //fruit feature class - Modify schema needs _all_ fields to be included
        //in the schema, not just the new ones to be added.
        var fruit_fc_desc = new FeatureClassDescription(fruit_fc_def);

        var modified_fld_descs = new List<ArcGIS.Core.Data.DDL.FieldDescription>(
          fruit_fc_desc.FieldDescriptions);

        modified_fld_descs.Add(fruit_types_fld);
        modified_fld_descs.Add(shelf_life_fld);

        //Create a feature class description to modify the fruit entity
        //with the new fields and their associated domains
        var updated_fruit_fc =
          new FeatureClassDescription(entity_name, modified_fld_descs,
                                      fruit_fc_desc.ShapeDescription);

        //Add the modified fruit fc desc to the schema builder
        sb.Modify(updated_fruit_fc);

        //Run the schema builder
        try
        {
          if (!kg.ApplySchemaEdits(sb))
          {
            var err_msg = string.Join(",", sb.ErrorMessages.ToArray());
            System.Diagnostics.Debug.WriteLine($"Create domains error: {err_msg}");
          }
        }
        catch (Exception ex)
        {
          System.Diagnostics.Debug.WriteLine(ex.ToString());
        }
      }
    });