Azure table storage allows you to store different data structures in the same table, but the standard routines for getting data in and out require you to know that structure at compile time – as you have to define the entity (class) to be persisted.

There are some very clever implementations that use the dynamic object features of C# to create objects with run-time defined properties. But what if you just want to store “some additional data” in key/value pairs but you don’t really need them to be properties on an object? There’s nothing out of the box for that, but you can override that by altering the Read/WriteEntity methods on TableEntity.

Here’s a naive implementation (it assumes string data, doesn’t limit number of data items, etc.)

public class TestEntity : TableEntity {

	public TestEntity(string a, string b) {
		PartitionKey = a;
		RowKey = b;
		DataItems = new Dictionary<string, string>();
		DataItems.Add("foo", "bar");
		DataItems.Add("Jacob", "Xander");
	}

	public Dictionary<string, string> DataItems { get; set; }

	public override IDictionary<string, EntityProperty> WriteEntity(OperationContext operationContext) {
		var results = base.WriteEntity(operationContext);
		foreach (var item in DataItems) {
			results.Add("D_" + item.Key, new EntityProperty(item.Value));
		}
		return results;
	}

	public override void ReadEntity(IDictionary<string, EntityProperty> properties, OperationContext operationContext) {
		DataItems = new Dictionary<string, string>();

		foreach (var item in properties) {
			if (item.Key == "Timestamp") {
				Timestamp = item.Value.DateTimeOffsetValue.Value;
			} else if (item.Key == "RowKey") {
				RowKey = item.Value.StringValue;
			} else if (item.Key == "PartitionKey") {
				PartitionKey = item.Value.StringValue;
			} else if (item.Key.StartsWith("D_")) {
				string realKey = item.Key.Substring(2);
				ItemData[realKey] = item.Value.StringValue;
			}
		}
	}
}

First key thing to remember – Azure table storage can only store a total of 255 key/value pairs – or 252 custom ones once you’ve taken PartitionKey, etc. into account.

I’d welcome any comments, improvements, or just drop me a note if you’ve used this in your own project – thanks!