public sealed class Attribute : ArcGIS.Desktop.Framework.Contracts.PropertyChangedBase, System.ComponentModel.INotifyPropertyChanged  

QueuedTask.Run(() =>
{
  var firstFeatureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<ArcGIS.Desktop.Mapping.FeatureLayer>().FirstOrDefault();

  // create an instance of the inspector class
  var inspector = new ArcGIS.Desktop.Editing.Attributes.Inspector();

  // load the layer
  inspector.LoadSchema(firstFeatureLayer);

  // iterate through the attributes, looking at properties
  foreach (var attribute in inspector)
  {
    var fldName = attribute.FieldName;
    var fldAlias = attribute.FieldAlias;
    var fldType = attribute.FieldType;
    int idxFld = attribute.FieldIndex;
    var fld = attribute.GetField();
    var isNullable = attribute.IsNullable;
    var isEditable = attribute.IsEditable;
    var isVisible = attribute.IsVisible;
    var isSystemField = attribute.IsSystemField;
    var isGeometryField = attribute.IsGeometryField;
  }
});

//See "Change Annotation Text Graphic" for an alternative if TEXTSTRING is missing from the schema
await QueuedTask.Run(() =>
{
    //annoLayer is ~your~ Annotation layer...

    // use the inspector methodology
    var insp = new Inspector();
    insp.Load(annoLayer, oid);

    // make sure TextString attribute exists.
    //It is not guaranteed to be in the schema
    ArcGIS.Desktop.Editing.Attributes.Attribute att = insp.FirstOrDefault(a => a.FieldName == "TEXTSTRING");
    if (att != null)
    {
        insp["TEXTSTRING"] = "Hello World";

        //create and execute the edit operation
        EditOperation op = new EditOperation();
        op.Name = "Update annotation";
        op.Modify(insp);

        //OR using a Dictionary - again TEXTSTRING has to exist in the schema
        //Dictionary<string, object> newAtts = new Dictionary<string, object>();
        //newAtts.Add("TEXTSTRING", "hello world");
        //op.Modify(annoLayer, oid, newAtts);

        op.Execute();
    }
});