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"));