feat: add c# code in SelectionSort (#691)

Author: PeixiZ

basic/sorting/SelectionSort/README.md

@@ -163,6 +163,48 @@ fn main() {
 }
 ```
 
+### **C#**
+
+```cs
+using static System.Console;
+namespace Pro;
+public class Program
+{
+    public static void Main()
+    {
+        int[] test = new int[] {90, 12, 77, 9, 0, 2, 23, 23, 3, 57, 80};
+        SelectionSortNums(test);
+        foreach (var item in test)
+        {
+            WriteLine(item);
+        }
+    }
+    public static void SelectionSortNums(int[] nums)
+    {        
+        for (int initial = 0; initial < nums.Length; initial++)
+        {
+            for (int second_sort = initial; second_sort < nums.Length; second_sort++)
+            {
+                if (nums[initial] > nums[second_sort])
+                {
+                    swap(ref nums[initial], ref nums[second_sort]);
+                }
+            }
+        }
+
+    }
+
+     private static void swap(ref int compare_left, ref int compare_right)
+    {
+        int temp = compare_left;
+        compare_left = compare_right;
+        compare_right = temp;
+    }
+
+}
+
+```
+
 <!-- tabs:end -->
 
 ## 算法分析

basic/sorting/SelectionSort/SelectionSort.cs

@@ -0,0 +1,36 @@
+using static System.Console;
+namespace Pro;
+public class Program
+{
+    public static void Main()
+    {
+        int[] test = new int[] {90, 12, 77, 9, 0, 2, 23, 23, 3, 57, 80};
+        SelectionSortNums(test);
+        foreach (var item in test)
+        {
+            WriteLine(item);
+        }
+    }
+    public static void SelectionSortNums(int[] nums)
+    {        
+        for (int initial = 0; initial < nums.Length; initial++)
+        {
+            for (int second_sort = initial; second_sort < nums.Length; second_sort++)
+            {
+                if (nums[initial] > nums[second_sort])
+                {
+                    swap(ref nums[initial], ref nums[second_sort]);
+                }
+            }
+        }
+
+    }
+
+     private static void swap(ref int compare_left, ref int compare_right)
+    {
+        int temp = compare_left;
+        compare_left = compare_right;
+        compare_right = temp;
+    }
+
+}