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EntityFramework动态多条件查询与Lambda表达式树

2014-08-31 21:53 393 查看

在常规的信息系统中, 我们有需要动态多条件查询的情况, 例如UI上有多个选择项可供用户选择多条件查询数据.
那么在.net平台Entity Framework下, 我们用Lambda表达式树如何实现, 这里我们需要一个PredicateBuilder的UML类图:

实现的代码是这样的:

   /// <summary>


[code]    /// Enables the efficient, dynamic composition of query predicates.

/// </summary>

public static class PredicateBuilder

/// <summary>

/// Creates a predicate that evaluates to true.

    /// </summary>

public static Expression<Func<T, bool>> True<T>() { return param => true; }

/// <summary>

/// Creates a predicate that evaluates to false.

    /// </summary>

public static Expression<Func<T, bool>> False<T>() { return param => false; }

/// <summary>

/// Creates a predicate expression from the specified lambda expression.

    /// </summary>

public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }

/// <summary>

/// Combines the first predicate with the second using the logical "and".

    /// </summary>

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)

return first.Compose(second, Expression.AndAlso);

/// <summary>

/// Combines the first predicate with the second using the logical "or".

    /// </summary>

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)

return first.Compose(second, Expression.OrElse);

/// <summary>

/// Negates the predicate.

    /// </summary>

public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)

var negated = Expression.Not(expression.Body);

return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);

/// <summary>

/// Combines the first expression with the second using the specified merge function.

    /// </summary>

static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)

// zip parameters (map from parameters of second to parameters of first)

var map = first.Parameters

.Select((f, i) => new { f, s = second.Parameters[i] })

.ToDictionary(p => p.s, p => p.f);

// replace parameters in the second lambda expression with the parameters in the first

var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

// create a merged lambda expression with parameters from the first expression

return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);

/// <summary>

/// ParameterRebinder

    /// </summary>

class ParameterRebinder : ExpressionVisitor

    /// <summary>

/// The ParameterExpression map

/// </summary>

readonly Dictionary<ParameterExpression, ParameterExpression> map;

    /// <summary>

/// Initializes a new instance of the <see cref="ParameterRebinder"/> class.

/// </summary>

/// <param name="map">The map.</param>

ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)

this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();

    /// <summary>

/// Replaces the parameters.

/// </summary>

/// <param name="map">The map.</param>

/// <param name="exp">The exp.</param>

/// <returns>Expression</returns>

public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)

return new ParameterRebinder(map).Visit(exp);

    /// <summary>

/// Visits the parameter.

/// </summary>

/// <param name="p">The p.</param>

/// <returns>Expression</returns>

protected override Expression VisitParameter(ParameterExpression p)

ParameterExpression replacement;

if (map.TryGetValue(p, out replacement))

p = replacement;

return base.VisitParameter(p);

            var myProduct=pr.Repository.Find(


[code]                  BuildFindByAllQuery(productName, beignUpdateDate, endUpdateDate) ,

e => e.UpdatedTime,

pageIndex,

pageSize);

Assert.IsTrue(myProduct.Count>0);


 /// <summary>


[code]         /// Builds the find by all query.

     /// </summary>

private static Expression<Func<Product, bool>> BuildFindByAllQuery(string productName,DateTime? beignUpdateDate, DateTime? endUpdateDate)

var list = new List<Expression<Func<Product, bool>>>();

if (!string.IsNullOrEmpty(productName)) list.Add(c => c.ProductName == productName);

if (beignUpdateDate != null) list.Add(c => c.UpdatedTime >= beignUpdateDate);

if (endUpdateDate != null) list.Add(c => c.UpdatedTime <= endUpdateDate);

//Add more condition

Expression<Func<Product, bool>> productQueryTotal = null;

foreach (var expression in list)

productQueryTotal = expression.And(expression);

return productQueryTotal;

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

上面的方法中由三个条件动态组成, 一个是匹配productName, 另两个是beginUpdateDate与endUpdateDate. 在判断它们是否为时, 构建最终查询条件集合.

最后把结果传给某个Repository类, 完成相应的数据访问.

是不是很简单, 希望对您的软件开发有帮助.

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作者：Petter Liu

出处：http://www.cnblogs.com/wintersun/

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