使用代码
特别注意 ：拼接条件时，所使用到的条件strID，strBir必须是独立的
var predicate = PredicateBuilder.GetTrue<Student>();
predicate = predicate.And(it => it.id.ToString().Contains(strID));
predicate = predicate.And(it => it.Birthday.ToString().Contains(strBir));
predicate = predicate.And(it => it.Sex.ToString().Contains(strSex));
predicate = predicate.And(it => it.Age == 20);
var lst = db.Queryable<Student>.Where(predicate).ToList();
工具类
using System;
using System.Linq.Expressions;
namespace WindowsFormsApp1.Tools
{
public static class PredicateBuilder
{
public static Expression<Func<T, bool>> GetTrue<T>() { return f => true; }
public static Expression<Func<T, bool>> GetFalse<T>() { return f => false; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.AndAlso<T>(second, Expression.AndAlso);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.AndAlso<T>(second, Expression.OrElse);
}
private static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2, Func<Expression, Expression, BinaryExpression> func)
{
var parameter = Expression.Parameter(typeof(T));
var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
var left = leftVisitor.Visit(expr1.Body);
var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
var right = rightVisitor.Visit(expr2.Body);
return Expression.Lambda<Func<T, bool>>(
func(left, right), parameter);
}
private class ReplaceExpressionVisitor : ExpressionVisitor
{
private readonly Expression _oldValue;
private readonly Expression _newValue;
public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
{
_oldValue = oldValue;
_newValue = newValue;
}
public override Expression Visit(Expression node)
{
if (node == _oldValue)
return _newValue;
return base.Visit(node);
}
}
}
}
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