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

// Loads the schema of the specified feature layer and iterates through its attributes,  providing access to their properties such as field name, alias, type, and other metadata.
await QueuedTask.Run(() =>
{
  // create an instance of the inspector class
  var inspector = new Inspector();
  // load the layer
  inspector.LoadSchema(featureLayer);
  // 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;
  }
});

{
  var oid = 1;

  // See "Change Annotation Text Graphic" for an alternative if TEXTSTRING is missing from the schema

  // Note: QueuedTask is required to access the Inspector
  {
    // use the inspector methodology
    var insp = new Inspector();
    insp.Load(annotationLayer, 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(annotationLayer, oid, newAtts);

      op.Execute();
    }
  }
}