如何更改Entity Framework 6中字符串属性的默认最大长度？ [英] How can I change the default max length of string properties in Entity Framework 6?
问题描述
默认约定，实体模型中没有明确给出最大长度的字符串属性在数据库中设置为 nvarchar（max）。我们希望覆盖这个约定，并给出字符串的最大长度 nvarchar（100），如果它们还没有被明确设置。
By default convention, strings properties in an entity model that are not explicitly given a max length are set to nvarchar(max) in the database. We wish to override this convention and give strings a max length of nvarchar(100) if they are not already explicitly set otherwise.
我发现了 PropertyMaxLengthConvention 内置的约定，其描述和文档似乎是我正在寻找的。但是，它不工作，或者我使用它错了，或者它不做我认为它。
I discovered the PropertyMaxLengthConvention built-in convention, which by its description and documentation would seem to be what I am looking for. However, it either doesn't work or I'm using it wrong or it just doesn't do what I think it does.
我已经尝试只是添加约定：
I've tried simply adding the convention:
modelBuilder.Conventions.Add(new PropertyMaxLengthConvention(100));
然后我以为也许默认使用了，所以我尝试先删除它： p>
Then I thought maybe the default one is already being used, so I tried removing it first:
modelBuilder.Conventions.Remove<PropertyMaxLengthConvention>();
modelBuilder.Conventions.Add(new PropertyMaxLengthConvention(100));
我甚至尝试明确地在默认情况下添加约定：
I even tried explictly adding the convention before and after the default one:
modelBuilder.Conventions.AddBefore<PropertyMaxLengthConvention>(new PropertyMaxLengthConvention(100));
modelBuilder.Conventions.AddAfter<PropertyMaxLengthConvention>(new PropertyMaxLengthConvention(100));
没有喜悦。当我添加迁移时，列仍然创建为 nvarchar（max）。
No joy. When I add migrations, the columns are still created as nvarchar(max).
有没有办法使用那个惯例做我想要的？如果没有，我可以编写一个自定义约定，将默认字符串属性设置为 nvarchar（100），但仍然允许我明确地将它们设置为不同的值，包括maxlength？ p>
Is there a way to use that convention to do what I want? If not, can I write a custom convention that will default string properties to nvarchar(100) but will still allow me to explicitly set them to a different value including maxlength?
推荐答案
在跟踪上述约定的源代码后，我发现它只设置指定的属性的默认最大长度具有固定长度。 （奇怪！）
After tracking down the source code for the aforementioned convention, I discovered that it only sets the default max length for properties that are specified to have fixed length. (Bizarre!)
所以我拿起源代码并修改它来创建我自己的约定。现在，具有未指定最大长度的字符串属性将具有默认的最大长度，而不是nvarchar（max）。唯一的缺点是，在 IsMaxLength（）配置被明确应用的时候似乎没有一种方法来检测。所以如果我有一个我想要创建为nvarchar（max）的列，我不能使用 IsMaxLength（）来实现。
So I took the source code and modified it to create my own convention. Now string properties with unspecified max length will have a default max length instead of being nvarchar(max). The only downside is there doesn't appear to be a way to detect when the IsMaxLength() configuration is explicitly applied. So if I have a column that I do want to have created as nvarchar(max) I can't use IsMaxLength() to do it.
为了解决这个问题，我为StringPropertyConfiguration创建了一个名为 ForceMaxLength（）的扩展方法，该方法使用配置属性HasMaxLength（int .MaxValue） - 通常是无效的值，但我可以在我的习惯惯例中轻松测试。当我检测到它时，我只需将MaxLength设置为null，并将IsMaxLength设置为true，并使属性配置继续正常。
To address this, I created an extension method for StringPropertyConfiguration called ForceMaxLength() that configures the property with HasMaxLength(int.MaxValue) - ordinarily an invalid value, but one for which I can easily test in my custom convention. When I detect it, I simply set the MaxLength back to null and set the IsMaxLength to true and let the property configuration continue as normal.
以下是自定义约定： p>
Here's the custom convention:
using System;
using System.Collections.Generic;
using System.Data.Entity.Core.Metadata.Edm;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.ModelConfiguration.Conventions;
namespace MyProject.CustomConventions
{
public class CustomPropertyMaxLengthConvention : IConceptualModelConvention<EntityType>, IConceptualModelConvention<ComplexType>
{
private const int DefaultLength = 128;
private readonly int length;
public CustomPropertyMaxLengthConvention()
: this(DefaultLength)
{
}
public CustomPropertyMaxLengthConvention(int length)
{
if (length <= 0)
{
throw new ArgumentOutOfRangeException("length", "Invalid Max Length Size");
}
this.length = length;
}
public virtual void Apply(EntityType item, DbModel model)
{
SetLength(item.DeclaredProperties);
}
public virtual void Apply(ComplexType item, DbModel model)
{
SetLength(item.Properties);
}
private void SetLength(IEnumerable<EdmProperty> properties)
{
foreach (EdmProperty current in properties)
{
if (current.IsPrimitiveType)
{
if (current.PrimitiveType == PrimitiveType.GetEdmPrimitiveType(PrimitiveTypeKind.String))
{
SetStringDefaults(current);
}
if (current.PrimitiveType == PrimitiveType.GetEdmPrimitiveType(PrimitiveTypeKind.Binary))
{
SetBinaryDefaults(current);
}
}
}
}
private void SetStringDefaults(EdmProperty property)
{
if (property.IsUnicode == null)
{
property.IsUnicode = true;
}
SetBinaryDefaults(property);
}
private void SetBinaryDefaults(EdmProperty property)
{
if (property.MaxLength == int.MaxValue)
{
property.MaxLength = null;
property.IsMaxLength = true;
}
else if (property.MaxLength == null || !property.IsMaxLength)
{
property.MaxLength = length;
}
}
}
}
这是扩展方法：
using System.Data.Entity.ModelConfiguration.Configuration;
namespace MyProject.Model.Mapping
{
public static class MappingExtensions
{
public static void ForceMaxLength(this StringPropertyConfiguration obj)
{
obj.HasMaxLength(int.MaxValue);
}
}
}
使用方法如下： / p>
Here's how it's used:
using System.Data.Entity.ModelConfiguration;
namespace MyProject.Model.Mapping
{
public class MyEntityMap : EntityTypeConfiguration<MyEntity>
{
public MyEntityMap()
{
Property(v => v.StringValue).ForceMaxLength();
}
}
}
这篇关于如何更改Entity Framework 6中字符串属性的默认最大长度？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
