GH-41199: [C#] Fix accessing values of a sliced decimal array

csharp/src/Apache.Arrow/Arrays/Decimal128Array.cs

@@ -144,7 +144,7 @@ public Decimal128Array(ArrayData data)
  {
  return null;
  }
- return DecimalUtility.GetDecimal(ValueBuffer, index, Scale, ByteWidth);
+ return DecimalUtility.GetDecimal(ValueBuffer, Offset + index, Scale, ByteWidth);
  }
 
  public IList<decimal?> ToList(bool includeNulls = false)
@@ -177,7 +177,7 @@ public string GetString(int index)
  {
  return null;
  }
- return DecimalUtility.GetString(ValueBuffer, index, Precision, Scale, ByteWidth);
+ return DecimalUtility.GetString(ValueBuffer, Offset + index, Precision, Scale, ByteWidth);
  }
 
  public SqlDecimal? GetSqlDecimal(int index)
@@ -187,7 +187,7 @@ public string GetString(int index)
  return null;
  }
 
- return DecimalUtility.GetSqlDecimal128(ValueBuffer, index, Precision, Scale);
+ return DecimalUtility.GetSqlDecimal128(ValueBuffer, Offset + index, Precision, Scale);
  }
 
  int IReadOnlyCollection<SqlDecimal?>.Count => Length;

csharp/src/Apache.Arrow/Arrays/Decimal256Array.cs

@@ -151,7 +151,7 @@
  return null;
  }
 
- return DecimalUtility.GetDecimal(ValueBuffer, index, Scale, ByteWidth);
+ return DecimalUtility.GetDecimal(ValueBuffer, Offset + index, Scale, ByteWidth);
  }
 
  public IList<decimal?> ToList(bool includeNulls = false)
@@ -184,7 +184,7 @@
  {
  return null;
  }
- return DecimalUtility.GetString(ValueBuffer, index, Precision, Scale, ByteWidth);
+ return DecimalUtility.GetString(ValueBuffer, Offset + index, Precision, Scale, ByteWidth);
  }
 
  public bool TryGetSqlDecimal(int index, out SqlDecimal? value)
@@ -196,11 +196,11 @@
  }
 
  const int longWidth = 4;
- var span = ValueBuffer.Span.CastTo<long>().Slice(index * longWidth);
+ var span = ValueBuffer.Span.CastTo<long>().Slice((Offset + index) * longWidth);
  if ((span[2] == 0 && span[3] == 0) ||
  (span[2] == -1 && span[3] == -1))
  {
- value = DecimalUtility.GetSqlDecimal128(ValueBuffer, 2 * index, Precision, Scale);
+ value = DecimalUtility.GetSqlDecimal128(ValueBuffer, 2 * (Offset + index), Precision, Scale);
  return true;
  }
 
@@ -231,7 +231,7 @@
  }
 
  int IReadOnlyCollection<string>.Count => Length;
  string? IReadOnlyList<string>.this[int index] => GetString(index);
 
  IEnumerator<string> IEnumerable<string>.GetEnumerator()
  {

csharp/test/Apache.Arrow.Tests/Decimal128ArrayTests.cs

@@ -458,5 +458,48 @@ public void AppendRangeSqlDecimal()
  }
  }
  }
+
+ [Fact]
+ public void SliceDecimal128Array()
+ {
+ // Arrange
+ const int originalLength = 50;
+ const int offset = 3;
+ const int sliceLength = 32;
+
+ var builder = new Decimal128Array.Builder(new Decimal128Type(14, 10));
+ var random = new Random();
+
+ for (int i = 0; i < originalLength; i++)
+ {
+ if (random.NextDouble() < 0.2)
+ {
+ builder.AppendNull();
+ }
+ else
+ {
+ builder.Append(i * (decimal)Math.Round(random.NextDouble(), 10));
+ }
+ }
+
+ var array = builder.Build();
+
+ // Act
+ var slice = (Decimal128Array)array.Slice(offset, sliceLength);
+
+ // Assert
+ Assert.NotNull(slice);
+ Assert.Equal(sliceLength, slice.Length);
+ for (int i = 0; i < sliceLength; ++i)
+ {
+ Assert.Equal(array.GetValue(offset + i), slice.GetValue(i));
+ Assert.Equal(array.GetSqlDecimal(offset + i), slice.GetSqlDecimal(i));
+ Assert.Equal(array.GetString(offset + i), slice.GetString(i));
+ }
+
+ Assert.Equal(
+ array.ToList(includeNulls: true).Skip(offset).Take(sliceLength).ToList(),
+ slice.ToList(includeNulls: true));
+ }
  }
 }

csharp/test/Apache.Arrow.Tests/Decimal256ArrayTests.cs

@@ -476,5 +476,56 @@ public void AppendRangeSqlDecimal()
  }
  }
  }
+
+ [Fact]
+ public void SliceDecimal256Array()
+ {
+ // Arrange
+ const int originalLength = 50;
+ const int offset = 3;
+ const int sliceLength = 32;
+
+ var builder = new Decimal256Array.Builder(new Decimal256Type(14, 10));
+ var random = new Random();
+
+ for (int i = 0; i < originalLength; i++)
+ {
+ if (random.NextDouble() < 0.2)
+ {
+ builder.AppendNull();
+ }
+ else
+ {
+ builder.Append(i * (decimal)Math.Round(random.NextDouble(), 10));
+ }
+ }
+
+ var array = builder.Build();
+
+ // Act
+ var slice = (Decimal256Array)array.Slice(offset, sliceLength);
+
+ // Assert
+ Assert.NotNull(slice);
+ Assert.Equal(sliceLength, slice.Length);
+ for (int i = 0; i < sliceLength; ++i)
+ {
+ Assert.Equal(array.GetValue(offset + i), slice.GetValue(i));
+ if (array.TryGetSqlDecimal(offset + i, out var expectedSqlDecimal))
+ {
+ Assert.True(slice.TryGetSqlDecimal(i, out var actualSqlDecimal));
+ Assert.Equal(expectedSqlDecimal, actualSqlDecimal);
+ }
+ else
+ {
+ Assert.False(slice.TryGetSqlDecimal(i, out _));
+ }
+ Assert.Equal(array.GetString(offset + i), slice.GetString(i));
+ }
+
+ Assert.Equal(
+ array.ToList(includeNulls: true).Skip(offset).Take(sliceLength).ToList(),
+ slice.ToList(includeNulls: true));
+ }
  }
 }