[GitHub] [arrow] edponce commented on issue #12049: How to read arrow files data in C#

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edponce commented on issue #12049:
URL: https://github.com/apache/arrow/issues/12049#issuecomment-1006315899


   @neroarth94 There are several abstractions underlying the `Column` data structure. `var col` is a [`Column` which is represented by a `Field` and a `ChunkedArray`](https://github.com/apache/arrow/blob/master/csharp/src/Apache.Arrow/Column.cs#L27-L28). The `Data` getter gives you access to the [`ChunkedArray` which is a collection of `Arrays`](https://github.com/apache/arrow/blob/master/csharp/src/Apache.Arrow/ChunkedArray.cs#L28) that logically represent a single array of values. The `Array`s are the data structures that generally give you access to particular values. In your case, you have an [`Int32Array` which derives from `PrimitiveArray`](https://github.com/apache/arrow/blob/master/csharp/src/Apache.Arrow/Arrays/Int32Array.cs#L20) that contains the [`GetValue(index)`](https://github.com/apache/arrow/blob/master/csharp/src/Apache.Arrow/Arrays/PrimitiveArray.cs#L36) method for accessing a particular value.
   
   IIUC, unfortunately, the C# `ChunkedArray` API does not has an indexing method similar to [the one in the C++ API](https://github.com/apache/arrow/blob/master/cpp/src/arrow/chunked_array.cc#L149), so you would need to convert global `ChunkedArray` indices to and pair of `Array` index and relative value index. 
   
   For example, assuming the column consists of a single `ChunkedArray` you should be able to access the values as follows:
   ```c#
   var col = readBatch.Column(0);
   var value = col.Data.Array(0).GetValue(10);  // Value at index 10 is 15
   ```
   Note: Most of the data structures contain `Slice(offset, length)` methods that allow you to get a copy of a subset of the values.


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