GH-41136: [C#] Recompute null count for sliced arrays on demand (#41144)

### Rationale for this change

See #41136. This is required for writing sliced arrays to IPC files and streams. It should also be generally useful too by allowing users to be able to rely on valid null count values for sliced arrays.

### What changes are included in this PR?

Adds a new `ArrayData.GetNullCount()` method which computes the actual null count if it is unknown, and changes most uses of `ArrayData.NullCount` to `GetNullCount()`.

### Are these changes tested?

Yes, I've extended the existing slicing tests to also verify the null count, and added new tests specifically for union arrays, which needed special handling.

### Are there any user-facing changes?

Yes, this is a user-facing behaviour change. I don't expect that it should break any existing code as users are unlikely to rely on the NullCount being -1.

* GitHub Issue: #41136

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>

csharp/src/Apache.Arrow/Arrays/Array.cs

@@ -31,7 +31,7 @@ protected Array(ArrayData data)
 
  public int Offset => Data.Offset;
 
- public int NullCount => Data.NullCount;
+ public int NullCount => Data.GetNullCount();
 
  public ArrowBuffer NullBitmapBuffer => Data.Buffers[0];
 

csharp/src/Apache.Arrow/Arrays/ArrayData.cs

@@ -15,7 +15,6 @@
 
 using Apache.Arrow.Memory;
 using Apache.Arrow.Types;
-using Google.FlatBuffers;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -28,12 +27,30 @@ public sealed class ArrayData : IDisposable
 
  public readonly IArrowType DataType;
  public readonly int Length;
- public readonly int NullCount;
+
+ /// <summary>
+ /// The number of null values in the Array. May be -1 if the null count has not been computed.
+ /// </summary>
+ public int NullCount;
+
  public readonly int Offset;
  public readonly ArrowBuffer[] Buffers;
  public readonly ArrayData[] Children;
  public readonly ArrayData Dictionary; // Only used for dictionary type
 
+ /// <summary>
+ /// Get the number of null values in the Array, computing the count if required.
+ /// </summary>
+ public int GetNullCount()
+ {
+ if (NullCount == RecalculateNullCount)
+ {
+ NullCount = ComputeNullCount();
+ }
+
+ return NullCount;
+ }
+
  // This is left for compatibility with lower version binaries
  // before the dictionary type was supported.
  public ArrayData(
@@ -111,7 +128,25 @@ public ArrayData Slice(int offset, int length)
  length = Math.Min(Length - offset, length);
  offset += Offset;
 
- return new ArrayData(DataType, length, RecalculateNullCount, offset, Buffers, Children, Dictionary);
+ int nullCount;
+ if (NullCount == 0)
+ {
+ nullCount = 0;
+ }
+ else if (NullCount == Length)
+ {
+ nullCount = length;
+ }
+ else if (offset == Offset && length == Length)
+ {
+ nullCount = NullCount;
+ }
+ else
+ {
+ nullCount = RecalculateNullCount;
+ }
+
+ return new ArrayData(DataType, length, nullCount, offset, Buffers, Children, Dictionary);
  }
 
  public ArrayData Clone(MemoryAllocator allocator = default)
@@ -125,5 +160,24 @@ public ArrayData Clone(MemoryAllocator allocator = default)
  Children?.Select(b => b.Clone(allocator))?.ToArray(),
  Dictionary?.Clone(allocator));
  }
+
+ private int ComputeNullCount()
+ {
+ if (DataType.TypeId == ArrowTypeId.Union)
+ {
+ return UnionArray.ComputeNullCount(this);
+ }
+
+ if (Buffers == null || Buffers.Length == 0 || Buffers[0].IsEmpty)
+ {
+ return 0;
+ }
+
+ // Note: Dictionary arrays may be logically null if there is a null in the dictionary values,
+ // but this isn't accounted for by the IArrowArray.IsNull implementation,
+ // so we maintain consistency with that behaviour here.
+
+ return Length - BitUtility.CountBits(Buffers[0].Span, Offset, Length);
+ }
  }
 }

csharp/src/Apache.Arrow/Arrays/ArrayDataConcatenator.cs

@@ -71,7 +71,7 @@ public ArrayDataConcatenationVisitor(IReadOnlyList<ArrayData> arrayDataList, Mem
  foreach (ArrayData arrayData in _arrayDataList)
  {
  _totalLength += arrayData.Length;
- _totalNullCount += arrayData.NullCount;
+ _totalNullCount += arrayData.GetNullCount();
  }
  }
 

csharp/src/Apache.Arrow/Arrays/DenseUnionArray.cs

@@ -53,5 +53,28 @@ protected override bool FieldIsValid(IArrowArray fieldArray, int index)
  {
  return fieldArray.IsValid(ValueOffsets[index]);
  }
+
+ internal new static int ComputeNullCount(ArrayData data)
+ {
+ var offset = data.Offset;
+ var length = data.Length;
+ var typeIds = data.Buffers[0].Span.Slice(offset, length);
+ var valueOffsets = data.Buffers[1].Span.CastTo<int>().Slice(offset, length);
+ var childArrays = new IArrowArray[data.Children.Length];
+ for (var childIdx = 0; childIdx < data.Children.Length; ++childIdx)
+ {
+ childArrays[childIdx] = ArrowArrayFactory.BuildArray(data.Children[childIdx]);
+ }
+
+ var nullCount = 0;
+ for (var i = 0; i < length; ++i)
+ {
+ var typeId = typeIds[i];
+ var valueOffset = valueOffsets[i];
+ nullCount += childArrays[typeId].IsNull(valueOffset) ? 1 : 0;
+ }
+
+ return nullCount;
+ }
  }
 }

csharp/src/Apache.Arrow/Arrays/NullArray.cs

@@ -95,7 +95,7 @@ public NullArray(int length)
 
  public int Offset => Data.Offset;
 
- public int NullCount => Data.NullCount;
+ public int NullCount => Data.GetNullCount();
 
  public void Dispose() { }
  public bool IsNull(int index) => true;

csharp/src/Apache.Arrow/Arrays/SparseUnionArray.cs

@@ -47,5 +47,26 @@ protected override bool FieldIsValid(IArrowArray fieldArray, int index)
  {
  return fieldArray.IsValid(index);
  }
+
+ internal new static int ComputeNullCount(ArrayData data)
+ {
+ var offset = data.Offset;
+ var length = data.Length;
+ var typeIds = data.Buffers[0].Span.Slice(offset, length);
+ var childArrays = new IArrowArray[data.Children.Length];
+ for (var childIdx = 0; childIdx < data.Children.Length; ++childIdx)
+ {
+ childArrays[childIdx] = ArrowArrayFactory.BuildArray(data.Children[childIdx]);
+ }
+
+ var nullCount = 0;
+ for (var i = 0; i < data.Length; ++i)
+ {
+ var typeId = typeIds[i];
+ nullCount += childArrays[typeId].IsNull(offset + i) ? 1 : 0;
+ }
+
+ return nullCount;
+ }
  }
 }

csharp/src/Apache.Arrow/Arrays/UnionArray.cs

@@ -41,7 +41,7 @@ public abstract class UnionArray : IArrowArray
 
  public int Offset => Data.Offset;
 
- public int NullCount => Data.NullCount;
+ public int NullCount => Data.GetNullCount();
 
  public bool IsValid(int index) => NullCount == 0 || FieldIsValid(Fields[TypeIds[index]], index);
 
@@ -91,6 +91,16 @@ protected static void ValidateMode(UnionMode expected, UnionMode actual)
  }
  }
 
+ internal static int ComputeNullCount(ArrayData data)
+ {
+ return ((UnionType)data.DataType).Mode switch
+ {
+ UnionMode.Sparse => SparseUnionArray.ComputeNullCount(data),
+ UnionMode.Dense => DenseUnionArray.ComputeNullCount(data),
+ _ => throw new InvalidOperationException("unknown union mode in null count computation")
+ };
+ }
+
  private IReadOnlyList<IArrowArray> InitializeFields()
  {
  IArrowArray[] result = new IArrowArray[Data.Children.Length];

csharp/src/Apache.Arrow/C/CArrowArrayExporter.cs

@@ -115,7 +115,7 @@ private unsafe static void ConvertArray(ExportedAllocationOwner sharedOwner, Arr
  {
  cArray->length = array.Length;
  cArray->offset = array.Offset;
- cArray->null_count = array.NullCount;
+ cArray->null_count = array.NullCount; // The C Data interface allows the null count to be -1
  cArray->release = ReleaseArrayPtr;
  cArray->private_data = MakePrivateData(sharedOwner);
 

csharp/src/Apache.Arrow/Ipc/ArrowStreamWriter.cs

@@ -376,7 +376,7 @@ private void CreateSelfAndChildrenFieldNodes(ArrayData data)
  CreateSelfAndChildrenFieldNodes(data.Children[i]);
  }
  }
- Flatbuf.FieldNode.CreateFieldNode(Builder, data.Length, data.NullCount);
+ Flatbuf.FieldNode.CreateFieldNode(Builder, data.Length, data.GetNullCount());
  }
 
  private static int CountAllNodes(IReadOnlyList<Field> fields)

csharp/test/Apache.Arrow.Tests/ArrowArrayTests.cs

@@ -185,6 +185,7 @@ public void SlicePrimitiveArrayWithNulls()
  TestSlice<Date64Array, Date64Array.Builder>(x => x.Append(new DateTime(2019, 1, 1)).Append(new DateTime(2019, 1, 2)).AppendNull().Append(new DateTime(2019, 1, 3)));
  TestSlice<Time32Array, Time32Array.Builder>(x => x.Append(10).Append(20).AppendNull().Append(30));
  TestSlice<Time64Array, Time64Array.Builder>(x => x.Append(10).Append(20).AppendNull().Append(30));
+ TestSlice<Int32Array, Int32Array.Builder>(x => x.AppendNull().AppendNull().AppendNull()); // All nulls
 
  static void TestNumberSlice<T, TArray, TBuilder>()
  where T : struct, INumber<T>
@@ -314,6 +315,8 @@ private void ValidateArrays<T>(PrimitiveArray<T> slicedArray)
  .SequenceEqual(slicedArray.Values));
 
  Assert.Equal(baseArray.GetValue(slicedArray.Offset), slicedArray.GetValue(0));
+
+ ValidateNullCount(slicedArray);
  }
 
  private void ValidateArrays(BooleanArray slicedArray)
@@ -333,6 +336,8 @@ private void ValidateArrays(BooleanArray slicedArray)
 #pragma warning disable CS0618
  Assert.Equal(baseArray.GetBoolean(slicedArray.Offset), slicedArray.GetBoolean(0));
 #pragma warning restore CS0618
+
+ ValidateNullCount(slicedArray);
  }
 
  private void ValidateArrays(BinaryArray slicedArray)
@@ -347,6 +352,16 @@ private void ValidateArrays(BinaryArray slicedArray)
  .SequenceEqual(slicedArray.ValueOffsets));
 
  Assert.True(baseArray.GetBytes(slicedArray.Offset).SequenceEqual(slicedArray.GetBytes(0)));
+
+ ValidateNullCount(slicedArray);
+ }
+
+ private static void ValidateNullCount(IArrowArray slicedArray)
+ {
+ var expectedNullCount = Enumerable.Range(0, slicedArray.Length)
+ .Select(i => slicedArray.IsNull(i) ? 1 : 0)
+ .Sum();
+ Assert.Equal(expectedNullCount, slicedArray.NullCount);
  }
  }
  }

csharp/test/Apache.Arrow.Tests/UnionArrayTests.cs

@@ -25,6 +25,46 @@ public class UnionArrayTests
  [InlineData(UnionMode.Sparse)]
  [InlineData(UnionMode.Dense)]
  public void UnionArray_IsNull(UnionMode mode)
+ {
+ var (array, expectedNull) = BuildUnionArray(mode, 100);
+
+ for (var i = 0; i < array.Length; ++i)
+ {
+ Assert.Equal(expectedNull[i], array.IsNull(i));
+ Assert.Equal(!expectedNull[i], array.IsValid(i));
+ }
+ }
+
+ [Theory]
+ [InlineData(UnionMode.Sparse)]
+ [InlineData(UnionMode.Dense)]
+ public void UnionArray_Slice(UnionMode mode)
+ {
+ var (array, expectedNull) = BuildUnionArray(mode, 10);
+
+ for (var offset = 0; offset < array.Length; ++offset)
+ {
+ for (var length = 0; length < array.Length - offset; ++length)
+ {
+ var slicedArray = ArrowArrayFactory.Slice(array, offset, length);
+
+ var nullCount = 0;
+ for (var i = 0; i < slicedArray.Length; ++i)
+ {
+ // TODO: Shouldn't need to add offset in IsNull/IsValid calls,
+ // see https://github.com/apache/arrow/issues/41140
+ Assert.Equal(expectedNull[offset + i], slicedArray.IsNull(offset + i));
+ Assert.Equal(!expectedNull[offset + i], slicedArray.IsValid(offset + i));
+ nullCount += expectedNull[offset + i] ? 1 : 0;
+ }
+
+ Assert.True(nullCount == slicedArray.NullCount, $"offset = {offset}, length = {length}");
+ Assert.Equal(nullCount, slicedArray.NullCount);
+ }
+ }
+ }
+
+ private static (UnionArray array, bool[] isNull) BuildUnionArray(UnionMode mode, int length)
  {
  var fields = new Field[]
  {
@@ -34,7 +74,6 @@ public void UnionArray_IsNull(UnionMode mode)
  var typeIds = fields.Select(f => (int) f.DataType.TypeId).ToArray();
  var type = new UnionType(fields, typeIds, mode);
 
- const int length = 100;
  var nullCount = 0;
  var field0Builder = new Int32Array.Builder();
  var field1Builder = new FloatArray.Builder();
@@ -44,7 +83,7 @@ public void UnionArray_IsNull(UnionMode mode)
 
  for (var i = 0; i < length; ++i)
  {
- var isNull = i % 5 == 0;
+ var isNull = i % 3 == 0;
  expectedNull[i] = isNull;
  nullCount += isNull ? 1 : 0;
 
@@ -104,10 +143,6 @@ public void UnionArray_IsNull(UnionMode mode)
  ? new DenseUnionArray(type, length, children, typeIdsBuffer, valuesOffsetBuffer, nullCount)
  : new SparseUnionArray(type, length, children, typeIdsBuffer, nullCount);
 
- for (var i = 0; i < length; ++i)
- {
- Assert.Equal(expectedNull[i], array.IsNull(i));
- Assert.Equal(!expectedNull[i], array.IsValid(i));
- }
+ return (array, expectedNull);
  }
 }