diff --git a/csharp/0448-find-all-numbers-disappeared-in-an-array.cs b/csharp/0448-find-all-numbers-disappeared-in-an-array.cs
new file mode 100644
index 000000000..58b5c139e
--- /dev/null
+++ b/csharp/0448-find-all-numbers-disappeared-in-an-array.cs
@@ -0,0 +1,15 @@
+public class Solution {
+    public IList<int> FindDisappearedNumbers(int[] nums) {
+        for(int i = 0; i < nums.Length; i++) {
+            int index = Math.Abs(nums[i]) - 1;
+            nums[index] = -Math.Abs(nums[index]);
+        }
+        List<int> result = new List<int>();
+        for(int i = 0; i < nums.Length; i++) {
+            if(nums[i] > 0) {
+                result.Add(i + 1);
+            }
+        }
+        return result;
+    }
+}
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