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The Field Object

  • Άρθρο

Each Field object usually corresponds to a column in a database table. However, a Field can also represent a pointer to another Recordset, called a chapter. Exceptions, such as chapter columns, will be covered later in this guide.

Use the Value property of Field objects to set or return data for the current record. Depending on the functionality the provider exposes, some collections, methods, or properties of a Field object may not be available.

With the collections, methods, and properties of a Field object, you can do the following:

  • Return the name of a field by using the Name property.

  • View or change the data in the field by using the Value property. Value is the default property of the Field object.

  • Return the basic characteristics of a field by using the Type, Precision, and NumericScale properties.

  • Return the declared size of a field by using the DefinedSize property.

  • Return the actual size of the data in a given field by using the ActualSize property.

  • Determine what types of functionality are supported for a given field by using the Attributes property and Properties collection.

  • Manipulate the values of fields containing long binary or long character data by using the AppendChunk and GetChunk methods.

Resolve discrepancies in field values during batch updating by using the OriginalValue and UnderlyingValue properties, if the provider supports batch updates.

Describing a Field

The topics that follow will discuss properties of the Field object that represent information that describes the Field object itself - that is, metadata about the field. This information can be used to determine much about the schema of the Recordset. These properties include Type, DefinedSize and ActualSize, Name, and NumericScale and Precision.

Discovering the Data Type

The Type property indicates the data type of the field. The data type enumerated constants that are supported by ADO are described in DataTypeEnum in the ADO Programmer's Reference.

For floating point numeric types such adNumeric, you can obtain more information. The NumericScale property indicates how many digits to the right of the decimal point will be used to represent values for the Field. The Precision property specifies the maximum number of digits used to represent values for the Field.

Determining Field Size

Use the DefinedSize property to determine the data capacity of a Field object.

Use the ActualSize property to return the actual length of a Field object's value. For all fields, the ActualSize property is read-only. If ADO cannot determine the length of the Field object's value, the ActualSize property returns adUnknown.

The DefinedSize and ActualSize properties have different purposes. For example, consider a Field object with a declared type of adVarChar and a DefinedSize property value of 50, containing a single character. The ActualSize property value it returns is the length in bytes of the single character.

Determining Field Contents

The identifier of the column from the data source is represented by the Name property of the Field. The Value property of the Field object returns or sets the actual data content of the field. This is the default property.

To change the data in a field, set the Value property equal to a new value of the correct type. Your cursor type must support updates to change the contents of a field. Database validation is not done here in batch mode, so you will need to check for errors when you call UpdateBatch in such a case. Some providers also support the ADO Field object's UnderlyingValue and OriginalValue properties to assist you with resolving conflicts when you attempt to perform batch updates. For details about how to resolve such conflicts, see Editing Data.

Note

Recordset Field values cannot be set when appending new Fields to a Recordset. Rather, new Fields can be appended to a closed Recordset. Then the Recordset must be opened, and only then can values be assigned to these Fields.

Getting More Field Information

ADO objects have two types of properties: built-in and dynamic. To this point, only the built-in properties of the Field object have been discussed.

Built-in properties are those properties implemented in ADO and immediately available to any new object, using the MyObject.Property syntax. They do not appear as Property objects in an object's Properties collection.

Dynamic properties are defined by the underlying data provider, and appear in the Properties collection for the appropriate ADO object. For example, a property specific to the provider may indicate if a Recordset object supports transactions or updating. These additional properties will appear as Property objects in that Recordset object's Properties collection. Dynamic properties can be referenced only through the collection, using the syntax MyObject.Properties(0) or MyObject.Properties("Name").

You cannot delete either kind of property.

A dynamic Property object has four built-in properties of its own:

  • The Name property is a string that identifies the property.

  • The Type property is an integer that specifies the property data type.

  • The Value property is a variant that contains the property setting. Value is the default property for a Property object.

  • The Attributes property is a Long value that indicates characteristics of the property specific to the provider.

The Properties collection for the Field object contains additional metadata about the field. The contents of this collection vary depending upon the provider. The following code example examines the Properties collection of the sample Recordset introduced at the beginning of this section. It first looks at the contents of the collection. This code uses the OLE DB Provider for SQL Server, so the Properties collection contains information relevant to that provider.

'BeginFieldProps  
    Dim objProp As ADODB.Property  
  
    For intLoop = 0 To (objFields.Count - 1)  
        Debug.Print objFields.Item(intLoop).Name  
  
        For Each objProp In objFields(intLoop).Properties  
            Debug.Print vbTab & objProp.Name & " = " & objProp.Value  
        Next objProp  
    Next intLoop  
'EndFieldProps

Dealing with Binary Data

Use the AppendChunk method on a Field object to fill it with long binary or character data. In situations where system memory is limited, you can use the AppendChunk method to manipulate long values in portions rather than in their entirety.

If the adFldLong bit in the Attributes property of a Field object is set to True, you can use the AppendChunk method for that field.

The first AppendChunk call on a Field object writes data to the field, overwriting any existing data. Subsequent AppendChunk calls add to existing data. If you are appending data to one field and then you set or read the value of another field in the current record, ADO assumes that you are finished appending data to the first field. If you call the AppendChunk method on the first field again, ADO interprets the call as a new AppendChunk operation and overwrites the existing data. Accessing fields in other Recordset objects that are not clones of the first Recordset object will not disrupt AppendChunk operations.

Use the GetChunk method on a Field object to retrieve part or all of its long binary or character data. In situations where system memory is limited, you can use the GetChunk method to manipulate long values in portions, rather than in their entirety.

The data that a GetChunk call returns is assigned to variable. If Size is greater than the remaining data, the GetChunk method returns only the remaining data without padding variable with empty spaces. If the field is empty, the GetChunk method returns a null value.

Each subsequent GetChunk call retrieves data starting from where the previous GetChunk call left off. However, if you are retrieving data from one field and then set or read the value of another field in the current record, ADO assumes you have finished retrieving data from the first field. If you call the GetChunk method on the first field again, ADO interprets the call as a new GetChunk operation and starts reading from the beginning of the data. Accessing fields in other Recordset objects that are not clones of the first Recordset object will not disrupt GetChunk operations.

If the adFldLong bit in the Attributes property of a Field object is set to True, you can use the GetChunk method for that field.

If there is no current record when you use the GetChunk or AppendChunk method on a Field object, error 3021 (no current record) occurs.

For an example of using these methods to manipulate binary data, see the AppendChunk Method and GetChunk Method examples in the ADO Programmer's Reference.