diff --git a/csharp/1396-design-underground-system.cs b/csharp/1396-design-underground-system.cs
new file mode 100644
index 000000000..d69635922
--- /dev/null
+++ b/csharp/1396-design-underground-system.cs
@@ -0,0 +1,31 @@
+public class UndergroundSystem
+{
+	// key: id
+	// value: station, time
+	private Dictionary<int, (string, int)> checked_in = new();
+	// key: start and end station
+	// value: total time, count
+	private Dictionary<(string, string), (int, int)> backlog = new();
+	public UndergroundSystem() { }
+
+	public void CheckIn(int id, string stationName, int t)
+	{
+		checked_in[id] = (stationName, t);
+	}
+
+	public void CheckOut(int id, string stationName, int t)
+	{
+		(string station, int time) = checked_in[id];
+		if (!backlog.TryAdd((station, stationName), (t - time, 1)))
+		{
+			(int total_time, int count) = backlog[(station, stationName)];
+			backlog[(station, stationName)] = (total_time + (t - time), count + 1);
+		}
+	}
+
+	public double GetAverageTime(string startStation, string endStation)
+	{
+		(int total_time, int count) = backlog[(startStation, endStation)];
+		return (double)total_time / count;
+	}
+}
\ No newline at end of file