Programming Entity Framework - Cdn.oreilly.com

The first method builds a setter for the given property that includes code to perform
validation on the length of the field. It calls the second method which takes an attribute
name, such as MaxLength, and reads the metadata to return the value of that attribute,
for example, 50, so that the setter can build the proper validation code as well as a
helpful error message.
Some of the code uses .NET Reflection, but some of it uses features of Entity
Framework’s MetadataWorkspace which knows how to read the metadata files.
You will learn much more about the MetadataWorkspace in
Chapter 21.
For example, the code to return the attrib value uses the MetadataWorkspace
TypeUsage method to find the MaxLength attribute. If the MaxLength attribute is
found, the code first checks for three possible problems. If the MaxLength is empty, is
set to SQL Server’s “Max” (e.g., varchar(Max)), or is a binary (Byte) field, the
validation code is not written. Otherwise, the method builds up a string that will test the
value of the property being set against the maximum length value. If the validation fails,
an ArgumentException is thrown with a specific description of the problem. If
MaxLength is not found, an empty string is returned.
Example 13-12 shows the template function that will generate the validation code for
you.
Example 13-12. The T4 template code for generatingMaxLength validation
string MaxLengthValidation(EdmProperty prop)
{
var attrib=prop.TypeUsage.Facets.FirstOrDefault(p=>p.Name=="MaxLength");
if (attrib != null)
{
string aVal=GetAttributeValue(attrib);
if (aVal == "Max" | aVal=="" | prop.TypeUsage.EdmType.Name == "Binary")
return "";
else
{
return System.Environment.NewLine +
"if (value.Length > " + aVal + ") " + System.Environment.NewLine +
new ArgumentException(\"" + prop.Name +
" must be less than " + aVal +" characters\");" +
System.Environment.NewLine +
" else";
}
}
else
{
return "";
}
}
string GetAttributeValue(Facet attrib)
{
var aVal=attrib.Value;
return Convert.ToString(aVal);
}