Fix DataFrame NullCount property of StringDataFrameColumn (#7090)

* Fix DataFrame NullCount property of StringDataFrameColumn works incorrectly

* Rerun unit tests

src/Microsoft.Data.Analysis/DataFrameColumns/StringDataFrameColumn.cs

@@ -35,6 +35,7 @@ namespace Microsoft.Data.Analysis
                     buffer.Add(default);
                 }
             }
+            _nullCount = length;
         }
 
         public StringDataFrameColumn(string name, IEnumerable<string> values) : base(name, 0, typeof(string))
@@ -325,7 +326,6 @@ namespace Microsoft.Data.Analysis
                     if (mapIndex == null)
                     {
                         setBuffer[(int)index] = null;
-                        ret._nullCount++;
                         return mapIndex;
                     }
 
@@ -339,8 +339,8 @@ namespace Microsoft.Data.Analysis
                     int bufferLocalMapIndex = (int)(mapIndex - getBufferMinRange);
                     string value = getBuffer[bufferLocalMapIndex];
                     setBuffer[(int)index] = value;
-                    if (value == null)
-                        ret._nullCount++;
+                    if (value != null)
+                        ret._nullCount--;
 
                     return mapIndex;
                 });
@@ -357,13 +357,12 @@ namespace Microsoft.Data.Analysis
                     if (mapIndex == null)
                     {
                         setBuffer[(int)index] = null;
-                        ret._nullCount++;
                         return mapIndex;
                     }
                     string value = getBuffer[mapIndex.Value];
                     setBuffer[(int)index] = value;
-                    if (value == null)
-                        ret._nullCount++;
+                    if (value != null)
+                        ret._nullCount--;
 
                     return mapIndex;
                 });

test/Microsoft.Data.Analysis.Tests/PrimitiveDataFrameColumnTests.cs

@@ -55,7 +55,7 @@ namespace Microsoft.Data.Analysis.Tests
             Assert.Equal(0, strCol.NullCount);
 
             StringDataFrameColumn strCol1 = new StringDataFrameColumn("String1", 5);
-            Assert.Equal(0, strCol1.NullCount);
+            Assert.Equal(5, strCol1.NullCount);
 
             StringDataFrameColumn strCol2 = new StringDataFrameColumn("String", Enumerable.Range(0, 10).Select(x => x.ToString()));
             Assert.Equal(0, strCol2.NullCount);
@@ -255,7 +255,7 @@ namespace Microsoft.Data.Analysis.Tests
         }
 
         [Fact]
-        public void TestClone()
+        public void TestClone_PrimitiveColumn()
         {
             PrimitiveDataFrameColumn<int> intColumn = new PrimitiveDataFrameColumn<int>("Int1", new int?[] { 1, 2, 3, 4, null });
             var copy = intColumn.Clone();
@@ -263,11 +263,27 @@ namespace Microsoft.Data.Analysis.Tests
             Assert.Equal(intColumn.Name, copy.Name);
             Assert.Equal(intColumn.Length, copy.Length);
             Assert.Equal(intColumn.DataType, copy.DataType);
+            Assert.Equal(intColumn.NullCount, copy.NullCount);
 
             for (int i = 0; i < intColumn.Length; i++)
                 Assert.Equal(intColumn[i], copy[i]);
         }
 
+        [Fact]
+        public void TestClone_StringColumn()
+        {
+            var strColumn = new StringDataFrameColumn("Str", ["str1", "str2", "srt3", null]);
+            var copy = strColumn.Clone();
+
+            Assert.Equal(strColumn.Name, copy.Name);
+            Assert.Equal(strColumn.Length, copy.Length);
+            Assert.Equal(strColumn.DataType, copy.DataType);
+            Assert.Equal(strColumn.NullCount, copy.NullCount);
+
+            for (int i = 0; i < strColumn.Length; i++)
+                Assert.Equal(strColumn[i], copy[i]);
+        }
+
         [Fact]
         public void TestNotNullableColumnClone()
         {