Null safe way to get values from an IDataReader

问题:

(LocalVariable)ABC.string(Name)= (Idatareader)datareader.GetString(0);

this name value is coming from database.. what happening here is if this name value is null while reading it's throwing an exception?

I am manually doing some if condition here. I don't want to write a manual condition to check all my variables..

I am doing something like this now..

string abc =  (Idatareader)datareader.GetValue(0);
if(abc = null)
    //assiging null
else
    assiging abc value

is there something like can we write extension method for this? thanks

回答1:

Here is a couple extension methods that will nicely wrap up all of your concerns around retrieving strongly typed values from a data reader. If the value is DbNull the default of the type will be returned. In the case of string which is a class, a null will be returned. If the field was int, then 0 would be returned. Additionally, if you are expecting an int?, say from an nullable int field, null would be returned.

Specific Usage for Kumar's case:

string abc = datareader.GetValueOrDefault<string>(0);

General Usage

var name = GetValueOrDefault<string>(reader, "Name");

or

var name = reader.GetValueOrDefault<string>("Name");

or

var name = reader.GetValueOrDefault<string>(0);

Extension

public static class NullSafeGetter
{
   public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName)
   {
       int ordinal = row.GetOrdinal(fieldName);
       return row.GetValueOrDefault<T>(ordinal);
   }

   public static T GetValueOrDefault<T>(this IDataRecord row, int ordinal)
   {
       return (T)(row.IsDBNull(ordinal) ? default(T) : row.GetValue(ordinal));
   }
}

from http://skysanders.net/subtext/archive/2010/03/02/generic-nullsafe-idatarecord-field-getter.aspx

回答2:

Similar to @sky-sanders answer but less strict with conversions:

public static T Get<T>(this IDataRecord row, string fieldName)
{
    int ordinal = row.GetOrdinal(fieldName);
    return row.Get<T>(ordinal);
}

public static T Get<T>(this IDataRecord row, int ordinal)
{
    var value = row.IsDBNull(ordinal) ? default(T) : row.GetValue(ordinal);
    return (T)Convert.ChangeType(value, typeof(T));
}

回答3:

My solution is that:

private static T GetValue<T>(object o) {
    if (typeof(DBNull) != o.GetType()) {
        return (T) o;
    }
    return default(T);
}

When, Status = GetValue<string>(currentDataRow["status"])

回答4:

Combining top solutions and suggestions, here is a C# 6 arrow expression version with support for GetValue<T> and GetValueOrDefault<T> with optional default value parameters.

public static class DataRecordExtensions {
    /// <summary>
    /// Generically extracts a field value by name from any IDataRecord as specified type. Will throw if DNE.
    /// </summary>
    public static T GetValue<T>(this IDataRecord row, string fieldName)
        => row.GetValue<T>(row.GetOrdinal(fieldName));

    /// <summary>
    /// Generically extracts a field value by ordinal from any IDataRecord as specified type. Will throw if DNE.
    /// </summary>
    public static T GetValue<T>(this IDataRecord row, int ordinal)
        => (T)row.GetValue(ordinal);

    /// <summary>
    /// Generically extracts a field value by name from any IDataRecord as specified type. Will return default generic types value if DNE.
    /// </summary>
    public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName, T defaultValue = default(T))
        => row.GetValueOrDefault<T>(row.GetOrdinal(fieldName), defaultValue);

    /// <summary>
    /// Generically extracts a field value by ordinal from any IDataRecord as specified type. Will return default generic types value if DNE.
    /// </summary>
    public static T GetValueOrDefault<T>(this IDataRecord row, int ordinal, T defaultValue = default(T))
        => (T)(row.IsDBNull(ordinal) ? defaultValue : row.GetValue(ordinal));
}

回答5:

I'd use something like this:

string abc = (IDataReader)datareader.GetValue(0) ?? "Default";