Handle null ref exceptions in LINQ Expression call

Question

I am trying to create a generic Expression builder, which basically works fine as long as none the objects values is null.

My current code looks like this (StartsWith as an example):

                case FilterOperationTypes.StartsWith:
                {
                    ParameterExpression e = Expression.Parameter(typeof(T), "e");
                    PropertyInfo propertyInfo = typeof(T).GetProperty(field);
                    MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
                    ConstantExpression c = Expression.Constant(val, typeof(string));
                    MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
                    Expression call = Expression.Call(m, mi, c);                        
                    return Expression.Lambda<Func<T, bool>>(call, e);
                }

Let's assume field is the Property CustomerName. I understand that the actual final expression will be like:

e.CustomerName.StartsWith(val)

and if CustomerName is null it will, of course, fail to call the StartsWith Method, which is perfectly clear.

I have tried to do something like this:

                case FilterOperationTypes.StartsWith:
                {
                    ParameterExpression e = Expression.Parameter(typeof(T), "e");
                    PropertyInfo propertyInfo = typeof(T).GetProperty(field);
                    MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
                    ConstantExpression c = Expression.Constant(val, typeof(string));
                    MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
                    Expression call = Expression.IfThenElse(
                        Expression.Equal(m, Expression.Constant(null)),
                        Expression.Constant(null),
                        Expression.Call(m, mi, c));
                    //Expression.Call(m, mi, c);                        
                    return Expression.Lambda<Func<T, bool>>(call, e);
                }

But this produces a Expression of type 'System.Void' cannot be used for return type 'System.Boolean' Exception.

I am a little bit lost as of now. Maybe you guys can push me in the right direction.

Thanks a lot!

Answer 1

Got it working now. Big thanks to Servy for pushing me in the right direction.

The Expression.Condition was exactly what I needed to achieve my goal. Since the actual object and the object's property can be null at run time I had to nest two Conditions:

            PropertyInfo propertyInfo = typeof(T).GetProperty(fieldName);
        MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
        ConstantExpression c = Expression.Constant(comparisonValue, typeof(string));            
        MethodInfo mi = typeof(string).GetMethod(methodName, new Type[] { typeof(string) });

        Expression call = Expression.Condition(Expression.Equal(e, Expression.Constant(null)),
        Expression.NotEqual(e, Expression.Constant(null)),
        Expression.Condition(Expression.Equal(Expression.Property(e, fieldName), Expression.Constant(null)),
        Expression.NotEqual(e, Expression.Constant(null)), Expression.Call(m, mi, c)));
        return Expression.Lambda<Func<T, bool>>(call, e);

In case of a null value I decided to use

Expression.NotEqual(e, Expression.Constant(null))

to be sure to receive "false". I'm sure there is a nicer way to do this, but I could not think of one, yet.

The final Expression will look like this

{e => IIF((e == null), (e != null), IIF((e.CustomerName== null), (e != null), e.CustomerName.StartsWith("547")))}

As of now I am pretty happy with the solution, though I'll try to optimize it.

My current implementation and usage of the code looks like this. Maybe it helps one of you guys:

        Expression<Func<T, bool>> GetDetailEx<T>(string fieldName, object comparisonValue, string filterType)
    {
        var e = Expression.Parameter(typeof(T),"e");
        switch (GetFilterOperationType(filterType))
        {
            case FilterOperationTypes.Between:
                //Between is automatically translated in >= AND <= within the ExecuteDeepFilter-Method
                break;
            case FilterOperationTypes.GreaterThanOrEquals:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.GreaterThanOrEqual(
                            Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                            Expression.Constant(comparisonValue)),e);
                }
            case FilterOperationTypes.LessThanOrEquals:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.LessThanOrEqual(
                        Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                        Expression.Constant(comparisonValue)),e);
                }
            case FilterOperationTypes.GreaterThan:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.GreaterThan(
                        Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                        Expression.Constant(comparisonValue)), e);
                }
            case FilterOperationTypes.LessThan:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.LessThan(
                        Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                        Expression.Constant(comparisonValue)), e);
                }
            case FilterOperationTypes.NotEqual:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.NotEqual(
                        Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                        Expression.Constant(comparisonValue)), e);
                }
            case FilterOperationTypes.Equal:
                {
                    return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.Equal(
                        Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
                        Expression.Constant(comparisonValue)), e);         
                }
            case FilterOperationTypes.EndsWith:
                {
                    return GenerateGenericCallExpression<T>(fieldName, comparisonValue, "EndsWith",e);
                }
            case FilterOperationTypes.StartsWith:
                {
                    return GenerateGenericCallExpression<T>(fieldName, comparisonValue, "StartsWith",e);
                }
            case FilterOperationTypes.NotContains:
                {
                    return GenerateGenericCallExpression<T>(fieldName, comparisonValue, "Contains",e,false); ;
                }
            case FilterOperationTypes.Contains:
                {
                    return GenerateGenericCallExpression<T>(fieldName, comparisonValue, "Contains",e);
                }
            default:
                break;
        }
        return GenerateConditionalExpression<T>(fieldName, comparisonValue, Expression.Equal(
            Expression.Convert(Expression.Property(e, fieldName), Expression.Constant(comparisonValue).Type),
            Expression.Constant(comparisonValue)), e);
    }

    private Expression<Func<T, bool>> GenerateConditionalExpression<T>(string fieldName, object comparisonValue, Expression actualExpression, ParameterExpression e)
    {
        Expression call = Expression.Condition(Expression.Equal(e, Expression.Constant(null)),
        Expression.NotEqual(e, Expression.Constant(null)),
        Expression.Condition(Expression.Equal(Expression.Property(e, fieldName), Expression.Constant(null)),
        Expression.NotEqual(e, Expression.Constant(null)), actualExpression));
        return Expression.Lambda<Func<T, bool>>(call, e);
    }

    private Expression<Func<T, bool>> GenerateGenericCallExpression<T>(string fieldName, object comparisonValue, string methodName, ParameterExpression e, bool equals = true)
    {
        PropertyInfo propertyInfo = typeof(T).GetProperty(fieldName);
        MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
        ConstantExpression c = Expression.Constant(comparisonValue, typeof(string));            
        MethodInfo mi = typeof(string).GetMethod(methodName, new Type[] { typeof(string) });

        if (equals)
        {
            Expression call = Expression.Condition(Expression.Equal(e, Expression.Constant(null)),
            Expression.NotEqual(e, Expression.Constant(null)),
            Expression.Condition(Expression.Equal(Expression.Property(e, fieldName), Expression.Constant(null)),
            Expression.NotEqual(e, Expression.Constant(null)), Expression.Call(m, mi, c)));
            return Expression.Lambda<Func<T, bool>>(call, e);
        }
        else
        {
            Expression call = Expression.Condition(Expression.Equal(e, Expression.Constant(null)),
            Expression.NotEqual(e, Expression.Constant(null)),
            Expression.Condition(Expression.Equal(Expression.Property(e, fieldName), Expression.Constant(null)),
            Expression.NotEqual(e, Expression.Constant(null)), Expression.Not(Expression.Call(m, mi, c))));
            return Expression.Lambda<Func<T, bool>>(call, e);
        }
    }

Answer 2

You're looking for Expression.Condition, rather than IfThenElse, which represents the conditional operator, rather than an if/else statement. The conditional operator resolves to a value, since it's an expression, not a statement.