添加项目文件。

HYJigPro/Jig3PtArc.cs

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+using Autodesk.Revit.Attributes;
+using Autodesk.Revit.DB;
+using Autodesk.Revit.UI;
+
+using System;
+
+namespace HYJig
+{
+    [Regeneration(0)]
+    [Transaction(TransactionMode.Manual)]
+    public class Jig3PtArc : IExternalCommand
+    {
+        public Result Execute(ExternalCommandData cmdData, ref string message, ElementSet elements)
+        {
+            UIDocument activeUIDocument = cmdData.Application.ActiveUIDocument;
+            ViewAssist.ViewValidating(activeUIDocument.Document, activeUIDocument.ActiveView);
+            XArc3PtJig xarc3PtJig = new XArc3PtJig(activeUIDocument);
+            XYZ xyz = xarc3PtJig.PickStartPoint("请确定圆弧起点：");
+            Result result;
+            if (xyz == null)
+            {
+                xarc3PtJig.Dispose();
+                result = 0;
+            }
+            else
+            {
+                XYZ xyz2 = xarc3PtJig.PickEndPoint(xyz, "请确定圆弧终点：");
+                if (xyz2 == null)
+                {
+                    xarc3PtJig.Dispose();
+                    result = 0;
+                }
+                else
+                {
+                    XYZ xyz3 = xarc3PtJig.PickOnCurvePoint(xyz, xyz2, "请确定圆弧上的点：");
+                    if (xyz3 == null)
+                    {
+                        xarc3PtJig.Dispose();
+                        result = 0;
+                    }
+                    else
+                    {
+                        Transaction transaction = new Transaction(activeUIDocument.Document);
+                        transaction.Start("CreateCurve");
+                        Arc arc = Arc.Create(xyz, xyz2, xyz3);
+                        activeUIDocument.Document.Create.NewDetailCurve(activeUIDocument.ActiveView, arc);
+                        transaction.Commit();
+                        xarc3PtJig.Dispose();
+                        result = 0;
+                    }
+                }
+            }
+            return result;
+        }
+    }
+}