多条件拼接

第一种：

internal class ParameterReplacer : ExpressionVisitor
{
public ParameterReplacer(ParameterExpression paramExpr)
{
this.ParameterExpression = paramExpr;
}

public ParameterExpression ParameterExpression { get; private set; }

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

protected override Expression VisitParameter(ParameterExpression p)
{
return this.ParameterExpression;
}
}

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

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

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> exp_left, Expression<Func<T, bool>> exp_right)
{
var candidateExpr = Expression.Parameter(typeof(T), "candidate");
var parameterReplacer = new ParameterReplacer(candidateExpr);

var left = parameterReplacer.Replace(exp_left.Body);
var right = parameterReplacer.Replace(exp_right.Body);
var body = Expression.And(left, right);

return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
}

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> exp_left, Expression<Func<T, bool>> exp_right)
{
var candidateExpr = Expression.Parameter(typeof(T), "candidate");
var parameterReplacer = new ParameterReplacer(candidateExpr);

var left = parameterReplacer.Replace(exp_left.Body);
var right = parameterReplacer.Replace(exp_right.Body);
var body = Expression.Or(left, right);

return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
}
}

第二种写法：

public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
}

public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
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>>(Expression.AndAlso(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);
}
}
}