public static void ShowMessageBox( 
   IEnumerable<IGPMessage> messages,
   string content_header,
   GPMessageBoxStyle style,
   string window_title,
   string icon_source,
   ViewModelBase parentViewModel
)

var environment = Geoprocessing.MakeEnvironmentArray(overwriteoutput: true);

      var prj = Project.Current;
      var map = MapView.Active;

      var defaultGDB = Project.Current.DefaultGeodatabasePath;

      var featLayers = map.Map.Layers.OfType<FeatureLayer>();

      var targetLayer = featLayers.ElementAt(0);  // First layer in TOC
      var joinLayer = featLayers.ElementAt(1);    // Second layer in TOC

      var outputFeatureClass = @"C:/temp/outputFC3.shp";

      // Specify the field map in Spatial Join with target and join feature class/layers in the App
      // Run Spatial Join manually - then Copy the fieldmap string from the result in Geoprocessing history and paste it for the fieldmap parameter. 
      // in this example of fieldmap, FireStations is the name of join layer
      // FireStations layer has two numeric fileds (used in Fieldmap): TYPE and NUMBER - these two fields are used in the FiedlMap
      //
      var joinLayerName = joinLayer.Name;
      var fieldMap = "TYPE 'TYPE' true true false 4 Long 0 0,Count,#,{joinLayerName},TYPE,-1,-1;NUMBER 'NUMBER' true true false 4 Long 0 0,Max,#,{joinLayerName},NUMBER,-1,-1";

      var toolParameters = Geoprocessing.MakeValueArray(targetLayer, joinLayer, outputFeatureClass, "JOIN_ONE_TO_ONE", "KEEP_COMMON", fieldMap, "INTERSECT");

      GPExecuteToolFlags executeFlags = GPExecuteToolFlags.AddOutputsToMap | GPExecuteToolFlags.GPThread | GPExecuteToolFlags.AddToHistory | GPExecuteToolFlags.RefreshProjectItems;

      IGPResult gpResult = await Geoprocessing.ExecuteToolAsync("analysis.SpatialJoin", toolParameters, environment, null, null, executeFlags);

      Geoprocessing.ShowMessageBox(gpResult.Messages, "GP Messages", gpResult.IsFailed ? GPMessageBoxStyle.Error : GPMessageBoxStyle.Default);

// get the syntax of the tool from Python window or from tool help page
string in_features = @"C:\data\data.gdb\HighwaysWeb84";
      string out_features = @"C:\data\data.gdb\HighwaysUTM";
      var param_values = Geoprocessing.MakeValueArray(in_features, out_features);

      // crate the spatial reference object to pass as an argument to management.CopyFeatures tool
      var sp_ref = await QueuedTask.Run(() => {
          return SpatialReferenceBuilder.CreateSpatialReference(26911);    // UTM 83 11N: 26911
      });

      // set output coordinate system environment           
      var environments = Geoprocessing.MakeEnvironmentArray(outputCoordinateSystem: sp_ref);
      // set environments in the 3rd parameter
      var gp_result = await Geoprocessing.ExecuteToolAsync("management.CopyFeatures", param_values, environments, null, null, GPExecuteToolFlags.AddOutputsToMap);
      
      Geoprocessing.ShowMessageBox(gp_result.Messages, "Contents", GPMessageBoxStyle.Default, "Window Title");

      //return gp_result;

var gp_result = await Geoprocessing.ExecuteToolAsync("management.GetCount", Geoprocessing.MakeValueArray(@"C:\data\f.gdb\hello"));
      // this icon shows up left of content_header
      string icon_src = @"C:\data\Icons\ModifyLink32.png";
      Geoprocessing.ShowMessageBox(gp_result.Messages, "Content Header", GPMessageBoxStyle.Error, "Window Title", icon_src);

var environments = Geoprocessing.MakeEnvironmentArray(overwriteoutput: true);

      string toolName = "Snap_edit";  // or use edit.Snap

      // Snap tool takes multiple inputs each of which has
      // Three (3) parts: a feature class or layer, a string value and a distance
      // Each part is separated by a semicolon - you can get example of sytax from the tool documentation page
      var snapEnv = @"'C:/SnapProject/fgdb.gdb/line_1' END '2 Meters';'C:/SnapProject/fgdb.gdb/points_1' VERTEX '1 Meters';'C:/SnapProject/fgdb.gdb/otherline_1' END '20 Meters'";

      var infc = @"C:/SnapProject/fgdb.gdb/poly_1";

      var snapParams = Geoprocessing.MakeValueArray(infc, snapEnv);

      GPExecuteToolFlags tokens = GPExecuteToolFlags.RefreshProjectItems | GPExecuteToolFlags.GPThread | GPExecuteToolFlags.AddToHistory;

      IGPResult snapResult = await Geoprocessing.ExecuteToolAsync(toolName, parameters, environments, null, null, tokens);

      //return gpResult;

ArcGIS.Desktop.Core.Events.GPExecuteToolEvent.Subscribe(e =>
      {
          string id = e.ID;                   // Same as history ID
          if (e.IsStarting == false)  // Execute completed
              _ = e.GPResult.ReturnValue;
          System.Windows.MessageBox.Show("event triggered.");
      });
await Geoprocessing.ExecuteToolAsync("management.GetCount", Geoprocessing.MakeValueArray(@"c:\shape_file.shp"));