Custom Aggregates in SQL Server

/// <summary>
/// Calculates the product of numerical values
/// </summary>
[System.Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Microsoft.SqlServer.Server.Format.Native,
    IsInvariantToDuplicates = false, // receiving the same value again changes the result
    IsInvariantToNulls = false,      // receiving a NULL value changes the result
    IsInvariantToOrder = true,       // the order of the values doesn't affect the result
    IsNullIfEmpty = true,            // if no values are given the result is null
    Name = "Product"                 // name of the aggregate
    )]
public struct Product {

    /// <summary>
    /// Used to store the product
    /// </summary>
    public System.Data.SqlTypes.SqlDouble Result { get; private set; }

    /// <summary>
    /// Used to inform if the accumulation has received values
    /// </summary>
    public bool HasValue { get; private set; }

    /// <summary>
    /// Initializes a new Product for a group
    /// </summary>
    public void Init() {
        this.Result = System.Data.SqlTypes.SqlDouble.Null;
        this.HasValue = false;
    }

    /// <summary>
    /// Calculates the product of the previous values and the value received
    /// </summary>
    /// <param name="number">Value to include</param>
    public void Accumulate(System.Data.SqlTypes.SqlDouble number) {
        if (!this.HasValue) {
            // if this is the first value received
            this.Result = number;
        }
        else if (this.Result.IsNull) {
            //if the calculated value is null, stay that way
        }
        else if (number.IsNull) {
            //if the value received is null the result is null
            this.Result = System.Data.SqlTypes.SqlDouble.Null;
        }
        else {
            //multiply the values
            this.Result = System.Data.SqlTypes.SqlDouble.Multiply(this.Result, number);
        }
        this.HasValue = true;
    }

    /// <summary>
    /// Merges this group to another group instatiated for the calculation
    /// </summary>
    /// <param name="group"></param>
    public void Merge(Product group) {
        // Count the product only if the other group has values
        if (group.HasValue) {
            this.Result = System.Data.SqlTypes.SqlDouble.Multiply(this.Result, group.Result);
        }
    }

    /// <summary>
    /// Ends the calculation and returns the result
    /// </summary>
    /// <returns></returns>
    public System.Data.SqlTypes.SqlDouble Terminate() {
        return this.Result;
    }
}