How to Cast object[] to generic T in C# Cosmos DB Serialization

This blog provides a solution for casting an `object[]` to a generic `T` in C# when overriding the default Cosmos DB serializer for array deserialization.
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How to Cast object[] to Generic T in C# Cosmos DB Serialization

When working with Cosmos DB and its custom serializers, you may encounter challenges when deserializing an array of documents. The requirement to cast object[] to a generic type T often leads to type conversion errors. In this guide, we will understand the underlying problem and explore a solution to effectively handle this conversion in C# .

Understanding the Problem

When overriding the default Microsoft.Azure.Cosmos.CosmosSerializer, we aim to perform schema upgrades on documents while reading them from Cosmos DB. While this works well for individual documents, deserializing an array of documents can lead to complications:

Type Casting Issues: The method public override T FromStream<T>(Stream stream) is invoked to handle incoming data. However, when working with arrays, the compiler does not allow a direct conversion from object[] to T, especially when T is an array of a specific type.

Runtime Errors: Trying to force the cast using (T)(object)object[] results in runtime errors like Unable to cast object of type System.Object[] to type OurCustomType[].

This leads to the necessity for a more elegant solution in order to achieve the desired behavior.

The Solution: Using Array.CreateInstance

One of the effective strategies to overcome this obstacle is by utilizing the Array.CreateInstance method. This allows us to create an instance of the desired array type dynamically at runtime. Here’s how you can implement this solution.

Step-by-step Implementation

Update the Function Signature: Modify the function UpdateSchemaVersionToCurrent to return an Array instead of an object[].

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Adjust the Return Type: Ensure that your deserialization function handles the return value correctly, capturing the newly created array.

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By implementing the above adjustments, we avoid the type casting errors and allow for a proper handling of array types in your Cosmos DB serialization.

Conclusion

Casting an object[] to a generic type T in C# can present challenges, particularly in the context of Cosmos DB serialization. However, by using Array.CreateInstance, we can manage the deserialization process effectively, ensuring that our documents are processed accurately and efficiently.

Next time you need to handle customization in Cosmos DB serializers, remember this approach to simplify your coding journey!