Using SimpleInjector to Configure AutoMapper

Oct 13, 2025 by wbrum.org 130 views

Introduction

AutoMapper is a popular open-source library used for object-object mapping in .NET. It simplifies the process of converting between different data formats, such as converting a database entity to a view model or vice versa. However, when working with AutoMapper, you may encounter issues when mapping null values to strings using the ForAllPropertyMaps method. In this article, we will explore the issue and provide a solution to handle null to string mappings with AutoMapper.

Understanding the Issue

When using the ForAllPropertyMaps method with AutoMapper, you may encounter an AutoMapper.AutoMapperMappingException with the message 'Error mapping types.'. This exception occurs when AutoMapper is unable to map a type, which in this case is likely due to the null to string mapping.

To better understand the issue, let's take a look at an example. Suppose we have a simple entity class User with a Name property:

public class User
{
    public string Name { get; set; }
}

And a view model class UserViewModel with a Name property:

public class UserViewModel
{
    public string Name { get; set; }
}

We can use AutoMapper to map the User entity to the UserViewModel:

var user = new User { Name = "John Doe" };
var userViewModel = _mapper.Map<UserViewModel>(user);

However, if the Name property of the User entity is null, AutoMapper will throw an exception:

var user = new User { Name = null };
var userViewModel = _mapper.Map<UserViewModel>(user);
// AutoMapper.AutoMapperMappingException: 'Error mapping types.'

Configuring AutoMapper to Handle `null` to `string` Mappings

To handle null to string mappings with AutoMapper, we need to configure the mapper to ignore null values or provide a default value when mapping null to a string.

We can use the ForAllPropertyMaps method with the Configure method to configure the mapper:

var configExpression = new MapperConfigurationExpression();
configExpression.ForAllPropertyMaps((config, typeMap) =>
{
    if (typeMap.DestinationType == typeof(string))
    {
        config.ForMember(typeMap.DestinationProperty.Name, opt =>
        {
            opt.MapFrom(src => src == null ? string.Empty : src.ToString());
        });
    }
});

In this example, we're using the ForAllPropertyMaps method to iterate over all property maps and check if the destination type is a string. If it is, we're using the ForMember method to configure the mapper to map null values to an empty string.

Using SimpleInjector to Configure AutoMapper

If you're using SimpleInjector to configure your dependencies, you can use the Container instance to configure AutoMapper:

public IMapper GetMapper()
{
    var configExpression = new MapperConfigurationExpression();
    configExpression.ForAllPropertyMaps((config, typeMap) =>
    {
        if (typeMap.DestinationType == typeof(string))
        {
            config.ForMember(typeMap.DestinationProperty.Name, opt =>
            {
                opt.MapFrom(src => src == null ? string.Empty : src.ToString());
            });
        }
    });
    return _container.GetInstance<IMapper>();
}

In this example, we're using the Container instance to get an instance of the IMapper interface, which is configured using the MapperConfigurationExpression.

Conclusion

In this article, we've explored the issue of AutoMapper.AutoMapperMappingException: 'Error mapping types.' when using the ForAllPropertyMaps method with AutoMapper. We've also provided a solution to handle null to string mappings by configuring the mapper to ignore null values or provide a default value when mapping null to a string. By following the steps outlined in this article, you should be able to handle null to string mappings with AutoMapper and avoid the AutoMapper.AutoMapperMappingException.

Example Use Cases

Here are some example use cases for handling null to string mappings with AutoMapper:

Mapping null values to empty strings: When mapping null values to strings, you can use the ForMember method to configure the mapper to map null values to empty strings.
Mapping null values to default values: When mapping null values to strings, you can use the ForMember method to configure the mapper to map null values to default values.
Ignoring null values: When mapping null values to strings, you can use the ForMember method to configure the mapper to ignore null values.

Best Practices

Here are some best practices for handling null to string mappings with AutoMapper:

Use the ForMember method: When mapping null values to strings, use the ForMember method to configure the mapper.
Provide a default value: When mapping null values to strings, provide a default value to avoid AutoMapper.AutoMapperMappingException.
Ignore null values: When mapping null values to strings, ignore null values to avoid AutoMapper.AutoMapperMappingException.

Q: What is AutoMapper and why do I need it?

A: AutoMapper is a popular open-source library used for object-object mapping in .NET. It simplifies the process of converting between different data formats, such as converting a database entity to a view model or vice versa. You need AutoMapper when you want to map complex objects between different data formats.

Q: What is the `ForAllPropertyMaps` method and how does it work?

A: The ForAllPropertyMaps method is a method in AutoMapper that allows you to iterate over all property maps and configure them. It takes a lambda expression as a parameter, which is used to configure each property map.

Q: What is the `ForMember` method and how does it work?

A: The ForMember method is a method in AutoMapper that allows you to configure a specific property map. It takes two parameters: the name of the property and a lambda expression that defines the mapping.

Q: Why do I get an `AutoMapper.AutoMapperMappingException: 'Error mapping types.'` when using the `ForAllPropertyMaps` method?

A: You get an AutoMapper.AutoMapperMappingException: 'Error mapping types.' when using the ForAllPropertyMaps method because AutoMapper is unable to map a type. This is often due to a null value being mapped to a string.

Q: How do I handle `null` to `string` mappings with AutoMapper?

A: To handle null to string mappings with AutoMapper, you need to configure the mapper to ignore null values or provide a default value when mapping null to a string. You can use the ForMember method to configure the mapper.

Q: How do I configure AutoMapper to ignore `null` values?

A: To configure AutoMapper to ignore null values, you can use the ForMember method and set the NullSubstitute property to null.

Q: How do I configure AutoMapper to provide a default value when mapping `null` to a string?

A: To configure AutoMapper to provide a default value when mapping null to a string, you can use the ForMember method and set the NullSubstitute property to a default value.

Q: Can I use AutoMapper with SimpleInjector?

A: Yes, you can use AutoMapper with SimpleInjector. You can use the Container instance to get an instance of the IMapper interface, which is configured using the MapperConfigurationExpression.

Q: What are some best practices for using AutoMapper?

A: Some best practices for using AutoMapper include:

Use the ForMember method to configure property maps.
Provide a default value when mapping null to a string.
Ignore null values when mapping null to a string.
Use the ForAllPropertyMaps method to iterate over all property maps.

By following these best practices and using the solution outlined in this article, you should be able to handle null to string mappings with AutoMapper and avoid the AutoMapper.AutoMapperMappingException.

Example Use Cases

Here are some example use cases for handling null to string mappings with AutoMapper:

Mapping null values to empty strings: When mapping null values to strings, you can use the ForMember method to configure the mapper to map null values to empty strings.
Mapping null values to default values: When mapping null values to strings, you can use the ForMember method to configure the mapper to map null values to default values.
Ignoring null values: When mapping null values to strings, you can use the ForMember method to configure the mapper to ignore null values.