public static class ExpressionEx
{
public static Expression<Func<T, bool>> True<T>() { return param => true; }
public static Expression<Func<T, bool>> False<T>() { return param => false; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)
{
return one.Compose(another, Expression.AndAlso);
//return one.Compose(another, Expression.And);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)
{
//return one.Compose(another, Expression.OrElse);
return one.Compose(another, Expression.Or);
}
public static Expression<Func<T, bool>> Compose<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another, Func<Expression, Expression, Expression> action)
{
//创建新参数
var newParameter = Expression.Parameter(typeof(T));
var parameterReplacer = new ParameterReplacer(newParameter);
var left = parameterReplacer.Replace(one.Body);
var right = parameterReplacer.Replace(another.Body);
var body = action(left, right);
return Expression.Lambda<Func<T, bool>>(body, newParameter);
}
/// <summary>
/// 参数替换
/// </summary>
internal class ParameterReplacer : ExpressionVisitor
{
public ParameterReplacer(ParameterExpression paramExpr)
{
Parameter = paramExpr;
}
//新的表达式参数
private ParameterExpression Parameter { get; set; }
public Expression Replace(Expression expr)
{
return this.Visit(expr);
}
/// <summary>
/// 覆盖父方法，返回新的参数
/// </summary>
/// <param name="p"></param>
/// <returns></returns>
protected override Expression VisitParameter(ParameterExpression p)
{
return Parameter;
}
}
}
