diff --git a/csharp/src/Apache.Arrow/Arrays/DenseUnionArray.cs b/csharp/src/Apache.Arrow/Arrays/DenseUnionArray.cs
index 14cfd9b0591b9..79880c894b13d 100644
--- a/csharp/src/Apache.Arrow/Arrays/DenseUnionArray.cs
+++ b/csharp/src/Apache.Arrow/Arrays/DenseUnionArray.cs
@@ -52,5 +52,28 @@ 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;
+        }
     }
 }
diff --git a/csharp/src/Apache.Arrow/Arrays/SparseUnionArray.cs b/csharp/src/Apache.Arrow/Arrays/SparseUnionArray.cs
index 1c1d1033d38a8..5b29489ebb1f0 100644
--- a/csharp/src/Apache.Arrow/Arrays/SparseUnionArray.cs
+++ b/csharp/src/Apache.Arrow/Arrays/SparseUnionArray.cs
@@ -46,5 +46,26 @@ 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;
+        }
     }
 }
diff --git a/csharp/src/Apache.Arrow/Ipc/ArrowStreamWriter.cs b/csharp/src/Apache.Arrow/Ipc/ArrowStreamWriter.cs
index b11479c0d4460..1cdd97e4ac6c9 100644
--- a/csharp/src/Apache.Arrow/Ipc/ArrowStreamWriter.cs
+++ b/csharp/src/Apache.Arrow/Ipc/ArrowStreamWriter.cs
@@ -557,6 +557,29 @@ public ArrowStreamWriter(Stream baseStream, Schema schema, bool leaveOpen, IpcOp
             }
         }
 
+        private void CreateSelfAndChildrenFieldNodes(ArrayData data)
+        {
+            if (data.DataType is NestedType)
+            {
+                // flatbuffer struct vectors have to be created in reverse order
+                for (int i = data.Children.Length - 1; i >= 0; i--)
+                {
+                    CreateSelfAndChildrenFieldNodes(data.Children[i]);
+                }
+            }
+            Flatbuf.FieldNode.CreateFieldNode(Builder, data.Length, data.GetNullCount());
+        }
+
+        private static int CountAllNodes(IReadOnlyList<Field> fields)
+        {
+            int count = 0;
+            foreach (Field arrowArray in fields)
+            {
+                CountSelfAndChildrenNodes(arrowArray.DataType, ref count);
+            }
+            return count;
+        }
+
         private Offset<Flatbuf.BodyCompression> GetBodyCompression()
         {
             if (_options.CompressionCodec == null)
diff --git a/csharp/test/Apache.Arrow.Tests/UnionArrayTests.cs b/csharp/test/Apache.Arrow.Tests/UnionArrayTests.cs
index 1fb5cf2415c68..45fed722a745c 100644
--- a/csharp/test/Apache.Arrow.Tests/UnionArrayTests.cs
+++ b/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);
     }
 }