LINQPad example of using StartsWith on RowKey and PartitionKey properties in Windows Azure Table Storage.

AzureTableStartsWith.cs

void Main()
{

    // Windows Azure Table Storage does not support a StartsWith function
    // on string properties. However, it does support the CompareTo function.
    // This will allow us to simulate the StartsWith function by taking the
    // substring that you want to match on and figuring out the exclusive
    // upper bound by incrementing the last character.

    // In this example, Buildings is a table that uses a postal code as
    // the partition key. The results would include the zip codes:
    // 19380, 19381, ... up through 19389.

    // This becomes:
    // (RowKey ge '1938') and (RowKey lt '1939')

    Buildings
        .WherePartitionKeyStartsWith("1938")
        .Take(100)
        .Dump();
    
}

public static class AzureTableExtensions {

    // Applies a predicate to the queryable that limits results to entities
    // that have a row key that begins with the specified substring.
    public static IQueryable<T> WhereRowKeyStartsWith<T>(this IQueryable<T> query, string startsWith) where T : ITableEntity {
        string upperBound = CreateUpperBoundString(startsWith);
        return query.Where( x => 
            x.RowKey.CompareTo(startsWith) >= 0 && 
            x.RowKey.CompareTo(upperBound) < 0 
        );
    }

    // Applies a predicate to the queryable that limits results to entities
    // that have a partition key that begins with the specified substring.
    public static IQueryable<T> WherePartitionKeyStartsWith<T>(this IQueryable<T> query, string startsWith) where T : ITableEntity, new() {
        string upperBound = CreateUpperBoundString(startsWith);
        return query.Where( x => 
            x.PartitionKey.CompareTo(startsWith) >= 0 && 
            x.PartitionKey.CompareTo(upperBound) < 0 
        );
    }

    // Given a string, returns a string that can be used as an upper bound in
    // a range query to achieve the same functionality as a StartsWith operator.
    // For example, passing ABCD as the lower bound will return ABCE as the upper
    // bound, so that the range would contain ABCD, ABCD0000, ABCDE, and everything
    // up until ABCE, which is the exclusive upper bound.
    private static string CreateUpperBoundString(string lowerBoundString) {
    
        if (String.IsNullOrEmpty(lowerBoundString)) {
            throw new ArgumentException("lowerBoundString parameter cannot be a null or empty string.", "lowerBoundString");
        }
    
        // increment the last character of the inclusive lower bound
        // to get the exclusive upper bound.
        string upperBoundString = lowerBoundString.Substring(0,lowerBoundString.Length-1) +
            (char)(lowerBoundString[lowerBoundString.Length-1] + 1);
    
        return upperBoundString;
        
    }

}