Comparing different types in Expression Trees

Question

In my API I give the option to filter the result - similar to the SQL statement WHERE. This works if I use a string field and compare this with a string value:

https://apiurl/items?filterfieldname=name&filterfieldvalue=test

Now I only get back items where the name is "test"

However, if I want to apply this to a bool field (in this case, called "isActive"), to return only active items:

https://apiurl/items?filterfieldname=isActive&filterfieldvalue=true

Then I get the following exception: System.InvalidOperationException: 'The binary operator Equal is not defined for the types 'System.Nullable`1[System.Boolean]' and 'System.String'.'

I use the following code to create the expression:

    static Expression<Func<T, bool>> GetExpressionForFilter<T>(string propertyName, string propertyValue)
    {
        var nameForWhereClause = propertyName.ToLowerInvariant();
        var valueForWhereClause = propertyValue.Trim().ToLowerInvariant();

        var parameterExp = Expression.Parameter(typeof(T), "type");
        var propertyExp = Expression.Property(parameterExp, nameForWhereClause);

        ConstantExpression someValue = Expression.Constant(valueForWhereClause, typeof(string));


        return Expression.Lambda<Func<T, bool>>(Expression.Equal(propertyExp, someValue), parameterExp);
    }

And apply the expression to my collection:

        var condition = GetExpressionForFilter<TEntity>(entitiesResourceParameters.FilterFieldName, entitiesResourceParameters.FilterFieldValue);
        condition.Compile();

        var collectionAfterFilter = collectionBeforeFilter.Where(condition).AsQueryable();

I tried to convert the property type to a string before comparing it to a "true" or "false" string, but this made no difference.

I would like to be able to enter any type of field (including boolean) and compare this with the value (as string) of this field (for example, "true" or "false"). Is this possible?

Answer 1

You can convert the string value into the type you're trying to compare it with.

    var propertyType = property.PropertyType;
    var value = Convert.ChangeType(valueForWhereClause, propertyType);

    ConstantExpression someValue = Expression.Constant(value, propertyType);

Be aware that Convert.ChangeType can throw an exception if an invalid value is provided (e.g. &filterfieldvalue=foo).