Linq To Entity 查询条件扩展

/// <summary>
/// 统一ParameterExpression
/// </summary>
internal class ParameterReplacer : ExpressionVisitor
{
public ParameterReplacer(ParameterExpression paramExpr)
{
ParameterExpression = paramExpr;
}

public ParameterExpression ParameterExpression { get; private set; }

public Expression Replace(Expression expr)
{
return Visit(expr);
}

protected override Expression VisitParameter(ParameterExpression p)
{
return ParameterExpression;
}
}
/// <summary>
/// Predicate扩展
/// </summary>
public static class PredicateExtensionses
{
/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> True<T>() { return f => true; }
/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> False<T>() { return f => false; }
/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="expLeft"></param>
/// <param name="expRight"></param>
/// <returns></returns>
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expLeft, Expression<Func<T, bool>> expRight)
{
var candidateExpr = Expression.Parameter(typeof(T), "candidate");
var parameterReplacer = new ParameterReplacer(candidateExpr);

var left = parameterReplacer.Replace(expLeft.Body);
var right = parameterReplacer.Replace(expRight.Body);
var body = Expression.And(left, right);

return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
}
/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="expLeft"></param>
/// <param name="expRight"></param>
/// <returns></returns>
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expLeft, Expression<Func<T, bool>> expRight)
{
var candidateExpr = Expression.Parameter(typeof(T), "candidate");
var parameterReplacer = new ParameterReplacer(candidateExpr);

var left = parameterReplacer.Replace(expLeft.Body);
var right = parameterReplacer.Replace(expRight.Body);
var body = Expression.Or(left, right);

return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
}
}
/// <summary>
/// Queryable扩展
/// </summary>
public static class QueryableExtensions
{
/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <param name="propertyName"></param>
/// <returns></returns>
public static IQueryable<T> OrderBy<T>(this IQueryable<T> queryable, string propertyName)
{
return OrderBy(queryable, propertyName, false);
}
/// <summary>
/// OrderBy
/// </summary>
/// <typeparam name="T">实体</typeparam>
/// <param name="queryable">条件</param>
/// <param name="propertyName">属性名称</param>
/// <param name="desc">是否降序</param>
/// <returns></returns>
public static IQueryable<T> OrderBy<T>(this IQueryable<T> queryable, string propertyName, bool desc)
{
var param = Expression.Parameter(typeof(T));
var body = Expression.Property(param, propertyName);
dynamic keySelector = Expression.Lambda(body, param);
return desc ? Queryable.OrderByDescending(queryable, keySelector) : Queryable.OrderBy(queryable, keySelector);
}
}