How do I upload to Azure Blob storage without overwriting?

Add an access condition to the code so that it checks against the ETag property of the blob - wildcards are allowed, so we want to only allow the upload if no blobs with this name have any etag (which is a roundabout way of saying, does this blob name exist).

You get a StorageException as detailed below.

    CloudBlockBlob blockBlob = container.GetBlockBlobReference(blobName);    try {        blockBlob.UploadFromStream(stream, accessCondition: AccessCondition.GenerateIfNoneMatchCondition("*"));    } catch (StorageException ex) {        if (ex.RequestInformation.HttpStatusCode == (int)System.Net.HttpStatusCode.Conflict) {            // Handle duplicate blob condition        }        throw;    }     

Azure now has an access condition that will only add, not overwrite: AccessCondition.GenerateIfNotExistsCondition()

Definition:

Constructs an access condition such that an operation will be performed only if the resource does not exist.

Example:

var accessCondition = AccessCondition.GenerateIfNotExistsCondition();blockBlob.UploadFromStream(stream, accessCondition);

The answer provided by Rob Church seems ok. Checking strings for errors isn't best practice and be improved with:

        CloudBlockBlob blockBlob = container.GetBlockBlobReference(blobName);        try        {            blockBlob.UploadFromStream(stream, accessCondition: AccessCondition.GenerateIfNoneMatchCondition("*"));        }        catch (StorageException ex)        {            if (ex.RequestInformation.HttpStatusCode == (int)System.Net.HttpStatusCode.Conflict)            {                // Handle duplicate blob condition            }            throw;        }