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ValueConverter Class

Serves as a base for the classes that represent property Value Converters.

Namespace: DevExpress.Xpo.Metadata

Assembly: DevExpress.Xpo.v19.1.dll

Declaration

public abstract class ValueConverter
Public MustInherit Class ValueConverter
public abstract class ValueConverter
Public MustInherit Class ValueConverter

Remarks

Value Converters can be used to persist properties that are non-persistent by default.

To implement a property value converter, create a class inherited from the ValueConverter class and override its abstract members.

The value converter can be associated either with a specific property by applying the ValueConverterAttribute attribute to the property or with a property type by calling the XPDictionary.RegisterValueConverter method of the metadata dictionary.

Note

The value converter mechanism is designed to allow persisting non-persistent data types by transforming them into a persistable equivalent. Value converters are not intended for introducing encryption, compression, or other business logic into the persistence process.

Important
  • Properties persisted using Value Converters require the client-side conversion logic, and should not be used in criteria expressions evaluated on the database server side. An attempt to use such properties in criteria may lead to undefined behavior. For instance, when a criterion is translated to a database query, a conversion implemented programmatically may not be applied or may be applied multiple times.

  • Value Converters cannot be applied to a property that represents a primary or foreign key. Use the approach described in the How to make user-friendly object identifiers Knowledge Base article if you need to use text values for object identifiers instead of auto-generated numbers.

If it is required to filter or sort by a value of a persistent type transformed from a stored persistent value, do not use value converters. Instead, create an additional calculated property decorated with PersistentAliasAttribute and express a conversion algorithm using the Criteria Language. If the algorithm is complex, implement a Custom Function Operator and use it in the criteria expression.

Examples

The following code example demonstrates how to store your boolean data in the database using the "T" string for true and "F" for false by defining a new ValueConverter descendant. You may need this converter when using an existing database in which boolean properties are stored such form.

//...       
[Size(1), ValueConverter(typeof(BooleanToStringValueConverter))]
public bool Answer {
    get { return GetPropertyValue<bool>("Answer"); }
    set { SetPropertyValue<bool>("Answer", value); }
}

//...
public class BooleanToStringValueConverter : ValueConverter {
    public override object ConvertFromStorageType(object value) {
        return Convert.ToString(value) == "T";
    }
    public override object ConvertToStorageType(object value) {
        return Convert.ToBoolean(value) ? "T" : "F";
    }
    public override System.Type StorageType {
        get { return typeof(string); }
    }
}

Inheritance

See Also