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Accepted
Raheel Khan
Asked: 2024-10-18 16:04:06 +0800 CST

检测并相应地转换 Linq 表达式

  • 772

我编写了一个扩展方法，以便更轻松地在 EF 8（.NET 8）中添加多列索引，如下所示：

public static IndexBuilder<TEntityType> HasIndexUnique<TEntityType>
(
    this EntityTypeBuilder<TEntityType> builder,
    string indexName,
    IEnumerable<Expression<Func<TEntityType, object?>>> includeExpressions
) where TEntityType : class
{
    if ((includeExpressions is null) || (!includeExpressions.Any()))
    {
        var currentMethod = MethodBase.GetCurrentMethod();
        throw (new Exception($@"The method [{currentMethod?.DeclaringType?.FullName}.{currentMethod?.Name}] was called without any [{nameof(includeExpressions)}]."));
    }

    var propertyNames = includeExpressions
        .Select
        (
            e =>
            // Casting issue depending on what was sent in.
            ((MemberExpression) ((UnaryExpression) e.Body).Operand).Member.Name
        )
        .ToArray();

    var indexBuilder = builder
        .HasIndex
        (
            propertyNames,
            $@"{indexName}"
        )
        .IsUnique(unique: true);

    return (indexBuilder);
}

的内置重载EntityTypeBuilder<TEntityType>.HasIndex是string基于的（我认为它很笨重）。支持语法的重载Expression仅允许一列。上述方法非常有效，使调用代码更易于阅读。

builder
    .HasIndexUnique
    (
        "Index_IndexName_Unique",
        e => e.ModelId,
        e => e.ModelEntityPrincipleId,
        e => e.ModelEntityDependentId,
        e => e.ModelEntityBridgeId
    );

在上面的例子中，传入的所有属性都是 类型Int64。当我尝试发送String属性时，表达式转换失败。

builder.HasIndexUnique("Index_IndexName_Unique", e => e.Name);

// Exception at this line:
((MemberExpression) ((UnaryExpression) e.Body).Operand).Member.Name
System.InvalidCastException: 'Unable to cast object of type 'System.Linq.Expressions.PropertyExpression' to type 'System.Linq.Expressions.UnaryExpression'.'

如何检测和处理不同类型的属性以便正确转换它们？我已经预料到以下几种数据类型，这应该足够了：

Int64, String, DateTime, Object, Byte []

我希望得到一个解决方案，但也想了解是什么使得表达式成为一元表达式、属性表达式或其他我尚未探索过的类型。

编辑：

public partial class ModelEntityRelationship:
    IEntity<ModelEntityRelationship>
{
    public virtual long Id { get; set; }
    public virtual string Name { get; set; }

    public virtual long ModelId { get; set; }
    public virtual Model Model { get; set; }

    public virtual long ModelEntityPrincipleId { get; set; }
    public virtual ModelEntity ModelEntityPrinciple { get; set; } = new();

    public virtual long ModelEntityDependentId { get; set; }
    public virtual ModelEntity ModelEntityDependent { get; set; } = new();

    public virtual long? ModelEntityBridgeId { get; set; }
    public virtual ModelEntity? ModelEntityBridge { get; set; }
}

我添加了源实体类来澄清我的困惑来源。除了数据类型之外，所有属性声明都是相同的。那么为什么在分别发送和对象时表达式类型Unary会Property从Int64变为String？

c#

1 个回答

  1. Best Answer

    我建议创建扩展方法来删除不需要的转换，通过转换为对象值类型来添加。

    public static class ExpressionExtensions
{
    [return: NotNullIfNotNull(nameof(ex))]
    public static Expression? UnwrapConvert(this Expression? ex)
    {
        if (ex == null)
            return null;

        switch (ex.NodeType)
        {
            case ExpressionType.ConvertChecked :
            case ExpressionType.Convert        :
            {
                var unaryExpression = (UnaryExpression)ex;
                if (unaryExpression.Method == null)
                    return unaryExpression.Operand.UnwrapConvert();
                break;
            }
        }

        return ex;
    }
}

    因此，对于您而言，您可以简单地转换为MemberExpression

    ((MemberExpression)e.Body.UnwrapConvert()).Member.Name
    • 1

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