How to get the maximum length of a string from an EDMX model in code?

问题

I've created an EDMX object from a database I'm programming against.

I need to get input from a user and save it to a row in the database table. The problem is that I need to limit the length of input strings to the width of the corresponding VARCHAR column in the database.

When I browse the model, I can clearly see in the properties window that the model knows the max length of the string, but I don't know how to access this data in code.

If I want to write something like this:

Entities entities = new Entities();
myTable = entities.myTable.First();
if (userInput.length > myTable.columnA.MaxLength)
{
    // tell the user that the input is too long.
}
else
{
    myTable.columnA = userInput;
}

How do I write it?

Update: I would like to point out that the IObjectContextAdapater mentioned in the answers below is in the System.Data.Entity.Infrastructure namespace.

回答1:

Here are two methods by which you can read the meta data:

int? GetMaxLength(DbContext context, string tableName, string propertyName)
{
    var oc = ((IObjectContextAdapter)context).ObjectContext;

    return oc.MetadataWorkspace.GetItems(DataSpace.CSpace).OfType<EntityType>()
             .Where(et => et.Name == tableName)
             .SelectMany(et => et.Properties.Where(p => p.Name == propertyName))
             .Select (p => p.MaxLength)
             .FirstOrDefault();
}

int? GetMaxLength<T>(DbContext context, Expression<Func<T, object>> property)
{
    var memberExpression = (MemberExpression)property.Body;
    string propertyName = memberExpression.Member.Name;
    return GetMaxLength(context, typeof(T).Name, propertyName);
}

So you can either enter the table name and property name, or an expression that specifies the property you're interested in.

Another approach could be to create a MetaData class and use the MaxLength attribute.

回答2:

It's not very pretty; reading edmx properties at runtime is not something Microsoft exposed easily or documented well (or in some cases, at all). context is your DBContext.

var objectContext = ((IObjectContextAdapter)context).ObjectContext;
var entityType = objectContext.MetadataWorkspace.GetItems<EntityType>(DataSpace.CSpace).Where(e => e.Name == "your entity name").First();
var facets = entityType.Properties["your property name"].TypeUsage.Facets;

facets will look something like this, so you'll need to look for the MaxLength Name(may not exist, depending on the underlying field type) and get the Value:

Count = 5
    [0]: Nullable=false
    [1]: DefaultValue=null
    [2]: MaxLength=250
    [3]: Unicode=false
    [4]: FixedLength=false

回答3:

If you modify the T4 template you can add your own attribute to the properties that have MaxLength set.

If you can find the right place to add it, it's something as simple as this:

var lengthAttributeText = edmProperty.MaxLength.HasValue
   ? string.Format("[MaxLength({0})] ", edmProperty.MaxLength.Value)
   : "";

And then add this into the text for the property line. (Tricky to be more detailed since I've already modified my .tt file a lot; also the lack of proper IDE support for .tt files makes this a lot harder than it could be.)

来源：https://stackoverflow.com/questions/27024764/how-to-get-the-maximum-length-of-a-string-from-an-edmx-model-in-code

标签

entity-framework

edmx