comparison of contracts vs traditional input checking.

gistfile1.cs

        private void CheckUsage(object owner, string operationId)
        {
            // New code
            Contract.Requires<InvalidOperationException>(owner == this.owner, "End was called on a different object than Begin");
            Contract.Requires<InvalidOperationException>(null != this.operationId, "End was called multiple times for this operation.");
            Contract.Requires<InvalidOperationException>(operationId == this.operationId, "End operation type was different than Begin.");

            // Old code
            if (!object.ReferenceEquals(owner, this.owner))
                throw new InvalidOperationException("End was called on a different object than Begin");
            else if (object.ReferenceEquals(null, this.operationId))
                throw new InvalidOperationException("End was called multiple times for this operation.");
            else if (!string.Equals(operationId, this.operationId))
                throw new InvalidOperationException("End operation type was different than Begin.");

            // cleanup
            this.operationId = null;
        }