AvnRepository

A flexible generic IRepository interface you can use on both the front end and back end. You can use it on the back end to access data, and also on the client to define a generic API Repository, which wraps calls to the API with an HttpClient object.

Nuget: https://www.nuget.org/packages/AvnRepository/

Classes

APIEntityResponse.cs

public class APIEntityResponse<TEntity> where TEntity : class
{
    public bool Success { get; set; }
    public List<string> ErrorMessages { get; set; } = new List<string>();
    public TEntity Data { get; set; }
}

APIListOfEntityResponse.cs

public class APIListOfEntityResponse<TEntity> where TEntity : class
{
    public bool Success { get; set; }
    public List<string> ErrorMessages { get; set; } = new List<string>();
    public IEnumerable<TEntity> Data { get; set; }
}

These two classes will be used as return types for our API controllers to add a little context to the actual entities returned.

IRepository.cs

using Microsoft.EntityFrameworkCore;
using System.Linq.Expressions;
using System.Reflection;
#nullable disable
/// <summary>
/// Generic repository interface that uses
/// </summary>
/// <typeparam name="TEntity"></typeparam>
public interface IRepository<TEntity> where TEntity : class
{
    Task<bool> DeleteAsync(TEntity EntityToDelete);
    Task<bool> DeleteByIdAsync(object Id);
    Task DeleteAllAsync(); // Be Careful!!!
    Task<IEnumerable<TEntity>> GetAsync(QueryFilter<TEntity> Filter);
    Task<IEnumerable<TEntity>> GetAllAsync();
    Task<TEntity> GetByIdAsync(object Id);
    Task<TEntity> InsertAsync(TEntity Entity);
    Task<TEntity> UpdateAsync(TEntity EntityToUpdate);
}

The IRepository<TEntity> interface will be used on the server as well as the client to ensure compatibility accessing data, no matter where the code resides.

This class file also includes code to describe custom queries that can easily be sent and received as JSON.

QueryFilter.cs

/// <summary>
/// A serializable filter. An alternative to trying to serialize and deserialize LINQ expressions,
/// which are very finicky. This class uses standard types. 
/// </summary>
/// <typeparam name="TEntity"></typeparam>
public class QueryFilter<TEntity> where TEntity : class
{
    /// <summary>
    /// If you want to return a subset of the properties, you can specify only
    /// the properties that you want to retrieve in the SELECT clause.
    /// Leave empty to return all columns
    /// </summary>
    public List<string> IncludePropertyNames { get; set; } = new List<string>();

    /// <summary>
    /// Defines the property names and values in the WHERE clause
    /// </summary>
    public List<FilterProperty> FilterProperties { get; set; } = new List<FilterProperty>();
    
    /// <summary>
    /// Specify the property to ORDER BY, if any 
    /// </summary>
    public string OrderByPropertyName { get; set; } = "";
    
    /// <summary>
    /// Set to true if you want to order DESCENDING
    /// </summary>
    public bool OrderByDescending { get; set; } = false;

    /// <summary>
    /// A custome query that returns a list of entities with the current filter settings.
    /// </summary>
    /// <param name="AllItems"></param>
    /// <returns></returns>
    public async Task<IEnumerable<TEntity>> GetFilteredListAsync(List<TEntity> AllItems)
    {
        // Convert to IQueryable
        var query = AllItems.AsQueryable<TEntity>();

        // the expression will be used for each FilterProperty
        Expression<Func<TEntity, bool>> expression = null;

        // Process each property
        foreach (var filterProperty in FilterProperties)
        {
            // use reflection to get the property info
            PropertyInfo prop = typeof(TEntity).GetProperty(filterProperty.Name);

            // string
            if (prop.PropertyType == typeof(string))
            {
                if (filterProperty.Operator == FilterOperator.Equals)
                    if (filterProperty.CaseSensitive == false)
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().ToLower() == filterProperty.Value.ToString().ToLower();
                    else
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString() == filterProperty.Value.ToString();
                else if (filterProperty.Operator == FilterOperator.NotEquals)
                    if (filterProperty.CaseSensitive == false)
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().ToLower() != filterProperty.Value.ToString().ToLower();
                    else
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString() != filterProperty.Value.ToString();
                else if (filterProperty.Operator == FilterOperator.StartsWith)
                    if (filterProperty.CaseSensitive == false)
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().ToLower().StartsWith(filterProperty.Value.ToString().ToLower());
                    else
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().StartsWith(filterProperty.Value.ToString());
                else if (filterProperty.Operator == FilterOperator.EndsWith)
                    if (filterProperty.CaseSensitive == false)
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().ToLower().EndsWith(filterProperty.Value.ToString().ToLower());
                    else
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().EndsWith(filterProperty.Value.ToString());
                else if (filterProperty.Operator == FilterOperator.Contains)
                    if (filterProperty.CaseSensitive == false)
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().ToLower().Contains(filterProperty.Value.ToString().ToLower());
                    else
                        expression = s => s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString().Contains(filterProperty.Value.ToString());
            }
            // int
            if (prop.PropertyType == typeof(int))
            {
                int value = Convert.ToInt32(filterProperty.Value);

                if (filterProperty.Operator == FilterOperator.Equals)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) == value;
                else if (filterProperty.Operator == FilterOperator.NotEquals)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) != value;
                else if (filterProperty.Operator == FilterOperator.LessThan)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) < value;
                else if (filterProperty.Operator == FilterOperator.GreaterThan)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) > value;
                else if (filterProperty.Operator == FilterOperator.LessThanOrEqual)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) <= value;
                else if (filterProperty.Operator == FilterOperator.GreaterThanOrEqual)
                    expression = s => Convert.ToInt32(s.GetType().GetProperty(filterProperty.Name).GetValue(s)) >= value;
            }
            // datetime
            if (prop.PropertyType == typeof(DateTime))
            {
                DateTime value = DateTime.Parse(filterProperty.Value);

                if (filterProperty.Operator == FilterOperator.Equals)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) == value;
                else if (filterProperty.Operator == FilterOperator.NotEquals)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) != value;
                else if (filterProperty.Operator == FilterOperator.LessThan)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) < value;
                else if (filterProperty.Operator == FilterOperator.GreaterThan)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) > value;
                else if (filterProperty.Operator == FilterOperator.LessThanOrEqual)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) <= value;
                else if (filterProperty.Operator == FilterOperator.GreaterThanOrEqual)
                    expression = s => DateTime.Parse(s.GetType().GetProperty(filterProperty.Name).GetValue(s).ToString()) >= value;
            }
            // Add expression creation code for other data types here.

            // apply the expression
            query = query.Where(expression);

        }

        // Include the specified properties
        foreach (var includeProperty in IncludePropertyNames)
        {
            query = query.Include(includeProperty);
        }

        // order by
        if (OrderByPropertyName != "")
        {
            PropertyInfo prop = typeof(TEntity).GetProperty(OrderByPropertyName);
            if (prop != null)
            {
                if (OrderByDescending)
                    query = query.Where(expression).OrderByDescending(x => prop.GetValue(x, null));
                else
                    query = query.Where(expression).OrderBy(x => prop.GetValue(x, null));
            }
        }

        // execute and return the list
        return query.ToList();
    }
}

The QueryFilter<TEntity> can be used on the client as well as the server to define the same level of filter as using LINQ, except that it easily travels across the wire.

• IncludedPropertyNames defines the columns to return, ala the SELECT clause
• FilterProperties defines the properties to compare, ala the WHERE clause
• OrderByPropertyName defines the sort column, ala the ORDER BY clause
• OrderByDescending defines the direction of the sort, ala DESC
• The GetFilteredList method applies the current filter settings given a list of all items. While it's true that all of the items need to be loaded, what you give up in memory efficiency you gain in convenience. This method currently handles properties of type string, int32 and DateTime.

FilterProperty.cs

/// <summary>
/// Defines a property for the WHERE clause
/// </summary>
public class FilterProperty
{
    public string Name { get; set; } = "";
    public string Value { get; set; } = "";
    public FilterOperator Operator { get; set; }
    public bool CaseSensitive { get; set; } = false;
}

The FilterProperty class defines columns to compare:

• Name is the Name of the property
• Value is a string representation of the value of the property
• CaseSensitive is a flag to determine whether case-sensitivity should be applied
• FilterOerator defines how to compare the column values (StartsWith, etc.)

FilterOperator.cs

/// <summary>
/// Specify the compare operator
/// </summary>
public enum FilterOperator
{
    Equals,
    NotEquals,
    StartsWith,
    EndsWith,
    Contains,
    LessThan,
    GreaterThan,
    LessThanOrEqual,
    GreaterThanOrEqual
}

Demo

I've created a demo that uses AvnRepository to create back end repositories for in-memory data, Entity Framework, and Dapper at https://github.com/carlfranklin/BlazorRepositoryDemo