public enum GPExecuteToolFlags : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

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;

// However, settings in Pro App's Geoprocessing Options will override option set in code
// for example, in Pro App's Options > Geoprocessing dialog, if you check 'Add output datasets to an open map'
// then the output WILL BE added to history overriding settings in code
var CopyfeaturesParams = Geoprocessing.MakeValueArray("C:\\data\\Input.gdb\\PlanA_Roads", "C:\\data\\Input.gdb\\Roads_copy");
IGPResult gpResult = await Geoprocessing.ExecuteToolAsync("management.CopyFeatures", CopyfeaturesParams, null, null, null, GPExecuteToolFlags.None);

// However, settings in Pro App's Geoprocessing Options will override option set in code
// for example, if in Options > Geoprocessing dialog, if you uncheck 'Write geoprocessing operations to Geoprocessing History'
// then the output will not be added to history.
var args2 = Geoprocessing.MakeValueArray("C:\\data\\Vegetation.shp", "NewField", "TEXT");
var result2 = await Geoprocessing.ExecuteToolAsync("management.AddField", args2, null, null, null, GPExecuteToolFlags.AddToHistory);

//The data referenced in this snippet can be downloaded from the arcgis-pro-sdk-community-samples repo
//https://github.com/Esri/arcgis-pro-sdk-community-samples
async Task<IGPResult> CreateRings(EditingTemplate currentTemplate)
{
  var paramsArray = Geoprocessing.MakeValueArray(currentTemplate.MapMember.Name,
              @"C:\Data\FeatureTest\FeatureTest.gdb\Points_MultipleRingBuffer",
              new List<string> { "1000", "2000" }, "Meters", "Distance",
              "ALL", "FULL");

  IGPResult ringsResult = await Geoprocessing.ExecuteToolAsync("Analysis.MultipleRingBuffer", paramsArray);
  var messages = string.IsNullOrEmpty(gpResult.ReturnValue)
          ? $@"Error in gp tool: {gpResult.ErrorMessages}"
          : $@"Ok: {gpResult.ReturnValue}";

  return ringsResult;
}

//The data referenced in this snippet can be downloaded from the arcgis-pro-sdk-community-samples repo
//https://github.com/Esri/arcgis-pro-sdk-community-samples
string in_data = @"C:\tools\data.gdb\cities";
string cities_buff = @"E:\data\data.gdb\cities_2km";

var valueArray = Geoprocessing.MakeValueArray(in_data, cities_buff, "2000 Meters");

// to let the GP tool run asynchronously without blocking the main thread
// use the GPThread option of GPExecuteToolFlasgs
//
GPExecuteToolFlags flags = GPExecuteToolFlags.GPThread;  // instruct the tool run non-blocking GPThread
IGPResult bufferResult = await Geoprocessing.ExecuteToolAsync("Analysis.Buffer", valueArray, null, null, null, flags);

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;

string openProjectPath = @"D\DATA\IGPHistoryItemTestProject\IGPHistoryItemTestProject.aprx";
await Project.OpenAsync(openProjectPath);
MapProjectItem mapProjItem = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(item => item.Name.Equals("Map", StringComparison.CurrentCultureIgnoreCase));

var map = await QueuedTask.Run(() => mapProjItem.GetMap());
var ftrLayer = map.Layers[0] as FeatureLayer;
string tool1 = "management.GetCount";
var args1 = Geoprocessing.MakeValueArray(ftrLayer);
var env = Geoprocessing.MakeEnvironmentArray(overwriteoutput: true);
GPExecuteToolFlags executeFlags = GPExecuteToolFlags.AddToHistory;
var t = await Geoprocessing.ExecuteToolAsync(tool1, args1, env, null, null, executeFlags);

IEnumerable<IGPHistoryItem> hisItems = Project.Current.GetProjectItemContainer(Geoprocessing.HistoryContainerKey) as IEnumerable<IGPHistoryItem>;

String hitemID = "";
String hitemToolPath = "";
IGPResult hitemGPResult = null;
DateTime hitemTimeStamp;

foreach (var hitem in hisItems)
{
  // common IGPHistoryItem and Item properties
  hitemID = (hitem as Item).ID;
  hitemToolPath = hitem.ToolPath;
  hitemGPResult = hitem.GPResult;
  hitemTimeStamp = hitem.TimeStamp;
}

var tool_name = "analysis.Clip";
var extent = MapView.Active.Extent;
var sel_layer = MapView.Active.Map.GetLayersAsFlattenedList()
                  .OfType<FeatureLayer>().FirstOrDefault(l => l.Name == "GreatLakes");
if (sel_layer == null) return;

var gdb = Project.Current.DefaultGeodatabasePath;
var out_fc = System.IO.Path.Combine(gdb, "clipped_lakes_out");

var val_array = await QueuedTask.Run(() =>
{

  var rect = GeometryEngine.Instance.Scale(extent, extent.Center, 0.5, 0.5) as Envelope;
  var clip_poly = PolygonBuilderEx.CreatePolygon(rect, rect.SpatialReference);

  //Add the geometry to a list before calling MakeValueArray
  //Envelope and Geometry types are supported
  var geom = new List<object>() { clip_poly };
  return Geoprocessing.MakeValueArray(new object[] { sel_layer, geom, out_fc });

});
Geoprocessing.ExecuteToolAsync(tool_name, val_array,
  null, null, null, GPExecuteToolFlags.InheritGPOptions);