Most efficient way to check for DBNull and then assign to a variable?

154

This question comes up occasionally, but I haven't seen a satisfactory answer.

A typical pattern is (row is a DataRow):

 if (row["value"] != DBNull.Value)
 {
      someObject.Member = row["value"];
 }

My first question is which is more efficient (I've flipped the condition):

  row["value"] == DBNull.Value; // Or
  row["value"] is DBNull; // Or
  row["value"].GetType() == typeof(DBNull) // Or... any suggestions?

This indicates that .GetType() should be faster, but maybe the compiler knows a few tricks I don't?

Second question, is it worth caching the value of row["value"] or does the compiler optimize the indexer away anyway?

For example:

  object valueHolder;
  if (DBNull.Value == (valueHolder = row["value"])) {}

Notes:

row["value"] exists.
I don't know the column index of the column (hence the column name lookup).
I'm asking specifically about checking for DBNull and then assignment (not about premature optimization, etc.).

I benchmarked a few scenarios (time in seconds, 10,000,000 trials):

row["value"] == DBNull.Value: 00:00:01.5478995
row["value"] is DBNull: 00:00:01.6306578
row["value"].GetType() == typeof(DBNull): 00:00:02.0138757

Object.ReferenceEquals has the same performance as "=="

The most interesting result? If you mismatch the name of the column by case (for example, "Value" instead of "value", it takes roughly ten times longer (for a string):

row["Value"] == DBNull.Value: 00:00:12.2792374

The moral of the story seems to be that if you can't look up a column by its index, then ensure that the column name you feed to the indexer matches the DataColumn's name exactly.

Caching the value also appears to be nearly

.net dbnull

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4 Answers

You should use the method:

Convert.IsDBNull()

Considering it's built-in to the Framework, I would expect this to be the most efficient.

I'd suggest something along the lines of:

int? myValue = (Convert.IsDBNull(row["column"]) ? null : (int?) Convert.ToInt32(row["column"]));

And yes, the compiler should cache it for you.

answered 2008-10-21T12:01:04.840

I would use the following code in C# (VB.NET is not as simple).

The code assigns the value if it is not null/DBNull, otherwise it asigns the default which could be set to the LHS value allowing the compiler to ignore the assign.

oSomeObject.IntMemeber = oRow["Value"] as int? ?? iDefault;
oSomeObject.StringMember = oRow["Name"] as string ?? sDefault;

answered 2009-04-14T20:27:17.963

This is how I handle reading from DataRows

///<summary>
/// Handles operations for Enumerations
///</summary>
public static class DataRowUserExtensions
{
    /// <summary>
    /// Gets the specified data row.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="dataRow">The data row.</param>
    /// <param name="key">The key.</param>
    /// <returns></returns>
    public static T Get<T>(this DataRow dataRow, string key)
    {
        return (T) ChangeTypeTo<T>(dataRow[key]);
    }

    private static object ChangeTypeTo<T>(this object value)
    {
        Type underlyingType = typeof (T);
        if (underlyingType == null)
            throw new ArgumentNullException("value");

        if (underlyingType.IsGenericType && underlyingType.GetGenericTypeDefinition().Equals(typeof (Nullable<>)))
        {
            if (value == null)
                return null;
            var converter = new NullableConverter(underlyingType);
            underlyingType = converter.UnderlyingType;
        }

        // Try changing to Guid  
        if (underlyingType == typeof (Guid))
        {
            try
            {
                return new Guid(value.ToString());
            }
            catch

            {
                return null;
            }
        }
        return Convert.ChangeType(value, underlyingType);
    }
}

Usage example:

if (dbRow.Get<int>("Type") == 1)
{
    newNode = new TreeViewNode
                  {
                      ToolTip = dbRow.Get<string>("Name"),
                      Text = (dbRow.Get<string>("Name").Length > 25 ? dbRow.Get<string>("Name").Substring(0, 25) + "..." : dbRow.Get<string>("Name")),
                      ImageUrl = "file.gif",
                      ID = dbRow.Get<string>("ReportPath"),


                        
                        
                            answered 2008-11-25T20:08:18.303


            
            
                
                    
                        
                            
                            
                        
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                        I've done something similar with extension methods. Here's my code:

public static class DataExtensions
{
    /// <summary>
    /// Gets the value.
    /// </summary>
    /// <typeparam name="T">The type of the data stored in the record</typeparam>
    /// <param name="record">The record.</param>
    /// <param name="columnName">Name of the column.</param>
    /// <returns></returns>
    public static T GetColumnValue<T>(this IDataRecord record, string columnName)
    {
        return GetColumnValue<T>(record, columnName, default(T));
    }

    /// <summary>
    /// Gets the value.
    /// </summary>
    /// <typeparam name="T">The type of the data stored in the record</typeparam>
    /// <param name="record">The record.</param>
    /// <param name="columnName">Name of the column.</param>
    /// <param name="defaultValue">The value to return if the column contains a <value>DBNull.Value</value> value.</param>
    /// <returns></returns>
    public static T GetColumnValue<T>(this IDataRecord record, string columnName, T defaultValue)
    {
        object value = record[columnName];
        if (value == null || value == DBNull.Value)
        {
            return defaultValue;
        }
        else
        {
            return (T)value;
        }
    }
}


To use it, you would do something like

int number = record.GetColumnValue<int>("Number",0)


                        
                        
                            answered 2008-11-26T01:22:53.160



        
        
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