Add new function: UnmergeCell (#13)

- Update unit test

Author: a3190778733-pixel

Excelize.Tests/UnitTest.cs

@@ -977,6 +977,14 @@ public void TestMergeCells()
         );
         RuntimeError err = Assert.Throws<RuntimeError>(() => f.MergeCell("SheetN", "A1", "B2"));
         Assert.Equal("sheet SheetN does not exist", err.Message);
+        Assert.Null(
+            Record.Exception(() =>
+            {
+                f.UnmergeCell("Sheet1", "D3", "E9");
+            })
+        );
+        err = Assert.Throws<RuntimeError>(() => f.UnmergeCell("SheetN", "A1", "B2"));
+        Assert.Equal("sheet SheetN does not exist", err.Message);
         Assert.Null(Record.Exception(() => f.SaveAs("TestMergeCells.xlsx")));
         Assert.Empty(f.Close());
     }

Excelize/Excelize.cs

@@ -421,6 +421,14 @@ internal static extern IntPtr MergeCell(
             [MarshalAs(UnmanagedType.LPUTF8Str)] string bottomRightCell
         );
 
+        [DllImport(LibraryName, CallingConvention = CallingConvention.Cdecl)]
+        internal static extern IntPtr UnmergeCell(
+            long fileIdx,
+            [MarshalAs(UnmanagedType.LPUTF8Str)] string sheet,
+            [MarshalAs(UnmanagedType.LPUTF8Str)] string topLeftCell,
+            [MarshalAs(UnmanagedType.LPUTF8Str)] string bottomRightCell
+        );
+
         [DllImport(LibraryName, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr MoveSheet(
             long fileIdx,
@@ -6338,6 +6346,37 @@ public void UngroupSheets()
                 throw new RuntimeError(err);
         }
 
+        /// <summary>
+        /// UnmergeCell provides a function to unmerge a given range reference.
+        /// <example>
+        /// For example unmerge range reference D3:E9 on Sheet1:
+        /// <code>
+        /// try
+        /// {
+        ///     f.UnmergeCell("Sheet1", "D3", "E9");
+        /// }
+        /// catch (RuntimeError err)
+        /// {
+        ///     Console.WriteLine(err.Message);
+        /// }
+        /// </code>
+        /// </example>
+        /// </summary>
+        /// <param name="sheet">The worksheet name</param>
+        /// <param name="topLeftCell">The top-left cell reference</param>
+        /// <param name="bottomRightCell">The bottom-right cell reference</param>
+        /// <remarks>Attention: overlapped range will also be unmerged</remarks>
+        /// <exception cref="RuntimeError">Return None if no error occurred,
+        /// otherwise raise a RuntimeError with the message.</exception>
+        public void UnmergeCell(string sheet, string topLeftCell, string bottomRightCell)
+        {
+            string err = Marshal.PtrToStringUTF8(
+                Lib.UnmergeCell(FileIdx, sheet, topLeftCell, bottomRightCell)
+            );
+            if (!string.IsNullOrEmpty(err))
+                throw new RuntimeError(err);
+        }
+
         /// <summary>
         /// UnsetConditionalFormat provides a function to unset the conditional
         /// format by given worksheet name and range reference.