Gets or sets a value indicating whether empty string ("") values are converted to null when the field values are returned from the data source.
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Sometimes the user may enter an empty string for a field value when updating or inserting records. Use the ImageField.ConvertEmptyStringToNull property to specify whether an empty string value is automatically converted to null when the data field is updated in the data source
The System.Web.UI.WebControls.ImageField class displays null as empty strings by default. To display a different value, set the ImageField.NullDisplayText property.