McRule/Ruler/Ruler.cs

using System.Linq;
using System.Linq.Expressions;

namespace Ruler;


public record FilterPolicy(
	string name, 
	string[] properties, 
	IEnumerable<(string, string)> scope, 
	FilterPolicyExtensions.RuleOperator ruleOperator=FilterPolicyExtensions.RuleOperator.And
	);

// Taken from LinqKit, Joseph Albahari. Man is a legend.
// LINQKit Copyright (c) 2007-2009 Joseph Albahari, Tomas Petricek
// 
// The Expression Visitor class is based on a Microsoft sample.
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

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 class FilterPolicyExtensions
{ 
    public enum RuleOperator { 
        And, 
        Or 
    } 

    public static Expression<Func<T, bool>> AddFilterToStringProperty<T>( 
                            Expression<Func<T, string>> expression, string filter, string filterType) 
    { 

#if DEBUG 
        if (!(filterType == "StartsWith" || filterType == "EndsWith" || filterType == "Contains")) 
        { 
            throw new Exception($"filterType must equal StartsWith, EndsWith or Contains. Passed {filterType}"); 
        } 

#endif
        // Check that the property isn't null, otherwise we'd hit null object exceptions at runtime
        var notNull = Expression.NotEqual(expression.Body,Expression.Constant(null)); 
         
        // Setup calls to EtartsWith, EndsWith, or Contains
        // TODO expressionArgs was used to pass multiple values for case insensitive compare, wasn't
        // mapping the method correctly when used with EF so need to revisit that
        var expressionArgs = new Expression[] {Expression.Constant(filter)}; 
        var strPredicate = Expression.Call(expression.Body, filterType, null, expressionArgs); 

        var filterExpression = Expression.AndAlso(notNull, strPredicate); 

        return Expression.Lambda<Func<T, bool>>( 
            filterExpression, 
            expression.Parameters); 
    } 

    // Dynamically build an expression suitable for filtering in a Where clause
    public static Expression<Func<T, bool>> GetFilterForType<T>(string property, string value) 
    { 
        var parameter = Expression.Parameter(typeof(T), "x"); 
        var opLeft = Expression.Property(parameter, property); 
        var opRight = Expression.Constant(value); 
        var comparison = Expression.Equal(opLeft, opRight); 

        // For string comparisons using wildcards, trim the wildcard characters and pass to the comparison method
        if (opLeft.Type == typeof(string)) { 
            // Grab the object property for use in the inner expression body
            var strParam = Expression.Lambda<Func<T,string>>(opLeft, parameter); 
             
            if (value.StartsWith("*") && value.EndsWith("*")) { 
                return AddFilterToStringProperty<T>(strParam, value.Trim('*'), "Contains"); 
            } else if (value.StartsWith("*")) { 
                return AddFilterToStringProperty<T>(strParam, value.TrimStart('*'), "EndsWith"); 
            } else if (value.EndsWith("*")) { 
                return AddFilterToStringProperty<T>(strParam, value.TrimEnd('*'), "StartsWith"); 
            } else { 
                return Expression.Lambda<Func<T, bool>>(comparison, parameter); 
            } 
        } 
         
        return Expression.Lambda<Func<T, bool>>(comparison, parameter); 
    } 

    // Combine a list of expressions inclusively
    public static Expression<Func<T, bool>>? CombineAnd<T>(IEnumerable<Expression<Func<T, bool>>> predicates) 
    { 
        if (predicates.Count() == 0) return null; 

        var final = predicates.First(); 
        foreach (var next in predicates.Skip(1)) 
            final = PredicateBuilder.And(final, next); 
     
        return final; 
    } 


    // Combine a list of expressions inclusively
    public static Expression<Func<T, bool>>? CombineOr<T>(IEnumerable<Expression<Func<T, bool>>> predicates) 
    { 
        if (predicates.Count() == 0) return null; 

        var final = predicates.First(); 
        foreach (var next in predicates.Skip(1)) 
            final = PredicateBuilder.Or(final, next); 

        return final; 
    } 


    // Combine a list of expressions inclusively
    public static Expression<Func<T, bool>>? CombinePredicates<T>(IEnumerable<Expression<Func<T, bool>>> predicates,
        FilterPolicyExtensions.RuleOperator op) 
    { 
        if (predicates.Count() == 0) return null; 

        if (op == RuleOperator.And) 
        { 
            return CombineAnd(predicates); 
        } 
        return CombineOr(predicates); 
    } 


    public static Expression<Func<T, bool>>? GetFilterExpression<T>(this FilterPolicy policy) 
    { 
        var predicates = new List<Expression<Func<T, bool>>>(); 
        foreach (var constraints in policy.scope) 
        { 
            predicates.Add(GetFilterForType<T>(constraints.Item1, constraints.Item2)); 
        } 
         
        return CombinePredicates<T>(predicates, policy.ruleOperator); 
    } 
}