Decoupling TableEntity while using Azure Storage

Azure Storage has a nice documentation here and is easy to get started with.

All the examples on the documentation mention that Entities on Azure Storage map to classes derived from TableEntity. However, this would not fit well with some of the design goals. This post is all about how to abstract this requirement to derive each class from TableEntity and keep everything tidy.

TableEntity class implements ITableEntity and it has two methods to read/write data to the object. We will be leveraging the same to come up with a solution.

Let’s create a base class, which we can use to derive all Entity classes.

public class StorageTableEntityBase
{
	public string ETag { get; set; }

	public string PartitionKey { get; set; }
	public string RowKey { get; set; }
	public DateTimeOffset Timestamp { get; set; }
	
	public StorageTableEntityBase()
	{
	
	}
	
	public StorageTableEntityBase(string partitionKey, string rowKey)
	{
		PartitionKey = partitionKey;
		RowKey = rowKey;
	}
}

PartitionKey, RowKey, Timestamp and ETag are the default properties each entity on Azure Storage possesses. StorageTableEntityBase class defined above lets us to contain these properties, copied over from the entity.

Let us create an adapter class which implements ITableEntity:

internal class AzStorageEntityAdapter<T> : ITableEntity where T : StorageTableEntityBase, new()
{
	/// <summary>
	/// Gets or sets the entity's partition key
	/// </summary>
	public string PartitionKey
	{
		get { return InnerObject.PartitionKey; }
		set { InnerObject.PartitionKey = value; }
	}

	/// <summary>
	/// Gets or sets the entity's row key.
	/// </summary>
	public string RowKey
	{
		get { return InnerObject.RowKey; }
		set { InnerObject.RowKey = value; }
	}

	/// <summary>
	/// Gets or sets the entity's Timestamp.
	/// </summary>
	public DateTimeOffset Timestamp
	{
		get { return InnerObject.Timestamp; }
		set { InnerObject.Timestamp = value; }
	}

	/// <summary>
	/// Gets or sets the entity's current ETag.
	/// Set this value to '*' in order to blindly overwrite an entity as part of an update operation.
	/// </summary>
	public string ETag
	{
		get { return InnerObject.ETag; }
		set { InnerObject.ETag = value; }
	}

	/// <summary>
	/// Place holder for the original entity
	/// </summary>
	public T InnerObject { get; set; } 

	public AzStorageEntityAdapter()
	{
		// If you would like to work with objects that do not have a default Ctor you can use (T)Activator.CreateInstance(typeof(T));
		this.InnerObject = new T();
	}

	public AzStorageEntityAdapter(T innerObject)
	{
		this.InnerObject = innerObject;
	} 

	public virtual void ReadEntity(IDictionary<string, EntityProperty> properties, OperationContext operationContext)
	{
		TableEntity.ReadUserObject(this.InnerObject, properties, operationContext);
	}

	public virtual IDictionary<string, EntityProperty> WriteEntity(OperationContext operationContext)
	{
		return TableEntity.WriteUserObject(this.InnerObject, operationContext);
	} 
}

I have marked the class as internal to limit its access to current assembly. This class copies over properties such as PartitionKey, implements ReadEntity and WriteEntity methods to copy over rest of the properties to a temporary object InnerObject.

We can now define our Entity Class as follows:

public class UserEntity : StorageTableEntityBase
	{
		public string UserName { get; set; }
		public string Email { get; set; }
	}

This is all the code we need to get it working. Now we can write a method to read from Storage Table as follows:

public T RetrieveEntity<T>(string tableName, string partitionKey, string rowKey)
        where T : StorageTableEntityBase, new()
{
    CloudTable table = TableClient.GetTableReference(tableName);
    TableResult tableResult = table.Execute(TableOperation.Retrieve<AzStorageEntityAdapter<T>>(partitionKey, rowKey));
    if (tableResult.Result != null)
    {
        return ((AzStorageEntityAdapter<T>)tableResult.Result).InnerObject;
    }
    return default(T);
}

I have not included the boilerplate methods instantiate TableClient, which is a trivial part and is available on the documentation.

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