PredicateBuilder.cs

using System;
using System.Linq.Expressions;

namespace MailSort;

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);
		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);
		return Expression.Lambda<Func<T, bool>>
			(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
	}
}