There are several kinds of fields in forms or datasheets that you can't edit. The following fields display values just as regular fields do, but if you try to enter data into them, nothing happens:

• AutoNumber fields    If a field in a Microsoft Access database (Microsoft Access database: A collection of data and objects (such as tables, queries, or forms) that is related to a particular topic or purpose. The Microsoft Jet database engine manages the data.) has an AutoNumber data type (AutoNumber data type: In a Microsoft Access database, a field data type that automatically stores a unique number for each record as it's added to a table. Three kinds of numbers can be generated: sequential, random, and Replication ID.) or in a Microsoft Access project (Microsoft Access project: An Access file that connects to a Microsoft SQL Server database and is used to create client/server applications. A project file doesn't contain any data or data-definition-based objects such as tables and views.) has a numeric data type with the Identity property set, Access automatically assigns a number to this field for each record you add. This type of field is often used as a record ID number or primary key (primary key: One or more fields (columns) whose values uniquely identify each record in a table. A primary key cannot allow Null values and must always have a unique index. A primary key is used to relate a table to foreign keys in other tables.). When you start adding a new record, Access automatically fills in the field's value for you and you can't edit it.
• Calculated fields    In an Access database, a calculated field (calculated field: A field, defined in a query, that displays the result of an expression rather than displaying stored data. The value is recalculated each time a value in the expression changes.) displays values that Access calculates. They are usually based on other fields in your tables, but calculated field values are not stored in tables.
• Locked or disabled fields    If a control on a form has the Locked property set to Yes or the Enabled property set to No, you can't edit the data in the field.
• Fields in a locked record    If you use an Access database in a multiuser environment and a record is locked by another user, you can't edit the data in the record. When you move to a locked record, the locked record indicator is displayed in the record selector.
• Fields in snapshots    While most queries return recordsets that you can edit, some queries return snapshots (snapshot: A static image of a set of data, such as the records displayed as the result of a query. Snapshot-type Recordset objects can be created from a base table, a query, or another recordset.), which can't be edited.
• Fields in a read-only form or a locked database If the form you're using has the AllowEdits property set to No or if the underlying data is read-only or locked by another person, you can't edit data in the form.

When you try to move out of a field after changing data, Microsoft Access validates the data, making sure that the value you entered is allowed in that field. If the value isn't allowed, Microsoft Access alerts you. In order to move off the field, you must either change the data to an acceptable value or undo your changes. If the record can't be saved, you must either make any necessary corrections or click Undo to cancel all changes you made to the record.

A value might not be allowed in a field for the following reasons:

• The value isn't compatible with the field's data type (data type: The characteristic of a field that determines what type of data it can hold. Data types include Boolean, Integer, Long, Currency, Single, Double, Date, String, and Variant (default).). If you enter a value that doesn't match the data type, Microsoft Access attempts to convert the entry to the correct data type. However, some data can't be converted to other data types. For example, you can't store a name in a field with the Number data type in a Microsoft Access database (Microsoft Access database: A collection of data and objects (such as tables, queries, or forms) that is related to a particular topic or purpose. The Microsoft Jet database engine manages the data.) or the smallint data type in a Microsoft Access project (Microsoft Access project: An Access file that connects to a Microsoft SQL Server database and is used to create client/server applications. A project file doesn't contain any data or data-definition-based objects such as tables and views.).
• You may be having problems entering dates in a field with the Date/Time data type. If you are entering dates using the slash mark (/) format, don't enter a trailing slash mark. For example, entering 1/3 is valid, but 1/3/ is not valid.
• The data breaks a rule set in the ValidationRule property for the field.
• Data is required in the field, and you've tried to leave it blank.
• The validation macro specified for the BeforeUpdate property canceled the update.
• The field does not accept zero-length strings (zero-length string: A string that contains no characters. You can use a zero-length string to indicate that you know no value exists for a field. You enter a zero-length string by typing two double quotation marks with no space between them (" ").). In an Access database, you can only enter a zero-length string in a Text or Memo field whose AllowZeroLength field property is set to Yes. In an Access project, you can only enter a zero-length string in a field with the varchar or nvarchar data type in a Microsoft SQL Server 7.x database. Zero-length strings are not supported in SQL Server 6.5.

• You tried to enter a duplicate value in a field that is a primary key (primary key: One or more fields (columns) whose values uniquely identify each record in a table. A primary key cannot allow Null values and must always have a unique index. A primary key is used to relate a table to foreign keys in other tables.) or that has a unique index (unique index: An index defined by setting a field's Indexed property to Yes (No Duplicates). A unique index will not allow duplicate entries in the indexed field. Setting a field as the primary key automatically defines the field as unique.).