Tài liệu Using the Get* Methods to Read Column Values doc

Using the Get* Methods to Read Column Values Before I show you the other Get* methods that read column values, you need to know the standard C# types and the values they support. You need to know these so that you can understand the type compatibilities between C# and SQL Server shown later. Table 9.3 shows the standard C# types, along with the underlying .NET type and the values that can be stored in the C# type. Table 9.3: STANDARD C# AND .NET TYPES C# TYPE .NET TYPE VALUES bool Boolean A Boolean true or false value. byte Byte An 8-bit unsigned integer between 0 and 2 8 - 1(255). char Char A 16-bit Unicode character. DateTime DateTime A date and time between 12:00:00 AM January 1, 0001 and 11:59:59 PM December 31, 9999. decimal Decimal A fixed precision and scale number between approximately +/-1.0 *10 -28 and approximately +/-7.9 *10 28 with 28 significant figures of precision. double Double A 64-bit floating-point number between approximately +/-5 *10 - 324 and approximately +/-1.7 *10 308 with 15 to 16 significant figures of precision. float Single A 32-bit floating-point number between approximately +/-1.5 *10 - 45 to approximately +/-3.4 *10 38 with 7 significant figures of precision. Guid Guid A 128-bit unsigned integer value (16 bytes) that that is unique across all computers and networks. int Int32 A 32-bit signed integer between -2 31 (-2,147,483,648) and 2 31 - 1 (2,147,483,647). long Int64 A 64-bit signed integer between -2 63 (- 9,223,372,036,854,775,808) and 2 63 - 1 (9,223,372,036,854,775,807). sbyte SByte An 8-bit signed integer between -2 7 (-128) and 2 7 - 1 (127). short Int16 A 16-bit signed integer between -2 15 (-32,768) and 2 15 - 1 (32,767). string String A variable-length string of 16-bit Unicode characters. Table 9.3: STANDARD C# AND .NET TYPES C# TYPE .NET TYPE VALUES uint UInt32 A 32-bit unsigned integer between 0 and 2 32 - 1 (4,294,967,295). ulong UInt64 A 64-bit unsigned integer between 0 and 2 64 - 1 (18,446,744,073,709,551,615). ushort UInt16 A 16-bit unsigned integer between 0 and 2 16 - 1 (65,535). Note The standard C# types are defined in the System namespace. Table 9.4 shows the SQL Server types, the compatible standard C# types, and the DataReader Get* methods that return each C# type. You use this table to figure out which method to call to get a specific column type. For example, if you need to get the value of a bigint column, you call the GetInt64() method that returns a long. Table 9.4: SQL SERVER TYPES, COMPATIBLE STANDARD C# TYPES, AND GET* METHODS SQL SERVER TYPE COMPATIBLE STANDARD C# TYPE GET* METHOD binary byte[] GetBytes() bigint long GetInt64() bit bool GetBoolean() char string GetString() datetime DateTime GetDateTime() decimal decimal GetDecimal() float double GetDouble() image byte[] GetBytes() int int GetInt32() money decimal GetDecimal() nchar string GetString() ntext string GetString() nvarchar string GetString() numeric decimal GetDecimal() real float GetFloat() smalldatetime DateTime GetDateTime() smallint short GetInt16() smallmoney decimal GetDecimal() Table 9.4: SQL SERVER TYPES, COMPATIBLE STANDARD C# TYPES, AND GET* METHODS SQL SERVER TYPE COMPATIBLE STANDARD C# TYPE GET* METHOD sql_varient object GetValue() text string GetString() timestamp byte[] GetBytes() tinyint byte GetByte() varbinary byte[] GetBytes() varchar string GetString() uniqueidentifier Guid GetGuid() Note You can see the SQL Server types and the values supported by those types in Table 2.3 of Chapter 2, "Introduction to Databases." Note The Get* methods are defined in all of the DataReader classes and work for all databases. Next you'll see how to use some of the methods shown in Table 9.4 . . Using the Get* Methods to Read Column Values Before I show you the other Get* methods that read column values, you need to know the standard. The standard C# types are defined in the System namespace. Table 9.4 shows the SQL Server types, the compatible standard C# types, and the DataReader Get*