diff --git a/cs/smallest-number-in-infinite-set.cs b/cs/smallest-number-in-infinite-set.cs
new file mode 100644
index 0000000..fb4570c
--- /dev/null
+++ b/cs/smallest-number-in-infinite-set.cs
@@ -0,0 +1,35 @@
+public class SmallestInfiniteSet {
+    private SortedSet<int> smaller;
+    private int nextInfinite;
+
+    public SmallestInfiniteSet() {
+        smaller = new SortedSet<int>();
+        nextInfinite = 1;
+    }
+
+    public int PopSmallest() {
+        var m = smaller.Min;
+        if (m != 0) {
+            smaller.Remove(m);
+            return m;
+        }
+
+        ++nextInfinite;
+        return nextInfinite - 1;
+    }
+
+    public void AddBack(int num) {
+        if (num >= nextInfinite) {
+            return;
+        }
+
+        smaller.Add(num);
+    }
+}
+
+/**
+ * Your SmallestInfiniteSet object will be instantiated and called as such:
+ * SmallestInfiniteSet obj = new SmallestInfiniteSet();
+ * int param_1 = obj.PopSmallest();
+ * obj.AddBack(num);
+ */